Fix default download directory and persist built-in aria2 sessions

[GT-610]

Summary

• Use the system Downloads folder as the default download directory when no config value exists, and normalize path joins to avoid malformed Windows paths.
• Align the built-in aria2 lifecycle with Motrix-style recovery by saving session state on terminal task transitions and shutting down through RPC so completed tasks are not reloaded on the next start.
• Add saveSession/shutdown RPC support for the built-in instance flow and normalize config/data file path handling.

Testing

• dart format on the modified Dart files.
• dart analyze on the modified Dart files: passed.
• flutter test test\download_task_service_test.dart: passed.
• flutter analyze: passed for the app code; one existing info-level lint remains in packages/fl_lib outside this change.

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Summary by CodeRabbit

• Bug Fixes

  • Fixed default download directory resolution and normalization across platforms; defaults are now computed asynchronously and persisted when missing or on load failure.

• Improvements

  • More reliable shutdown: attempts graceful RPC shutdown (with optional force) and waits before killing the process; improved session save behavior.
  • Session persistence optimized to save only for instances with terminal task changes.
  • Safer cross-platform path handling for configuration and downloads.

• New Features

  • Exposes instance process PID for diagnostics.
  • Added helpers to obtain the default download directory (sync and async).

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

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Configuration used: Organization UI

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Run ID: cacdc0d5-2559-4e73-94ab-291a0682e208

📥 Commits

Reviewing files that changed from the base of the PR and between 9ace276 and 99ff65c.

📒 Files selected for processing (2)

• lib/app.dart
• lib/services/builtin_instance_service.dart

✅ Files skipped from review due to trivial changes (2)

• lib/app.dart
• lib/services/builtin_instance_service.dart

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📝 Walkthrough

Walkthrough

Async default-download resolution added; path handling switched to package:path; Settings now normalizes and persists downloadDir defaults; Aria2 RPC gained a shutdown method; builtin instance stop attempts RPC save+shutdown with timed waits and kills; session saves are triggered for instances with tasks that transition to terminal states.

Changes

Instance lifecycle, settings, and session persistence

Layer / File(s)	Summary
Data Shape / Utilities lib/utils/default_download_directory.dart	Adds getDefaultDownloadDirectorySync() and async wrapper to resolve a platform-appropriate Downloads directory.
Settings defaults & file location lib/models/settings.dart	Makes default download-dir resolution async, adds _defaultDownloadDirectory(), normalizes downloadDir with p.normalize, builds settings path with p.join, and ensures defaults are persisted. _applyDefaultSettings became Future<void> and resetToDefaults() awaits default dir.
Core RPC extension lib/services/aria2_rpc_client.dart	Adds Future<bool> shutdown({bool force = false}) to call aria2.shutdown or aria2.forceShutdown, returning true when RPC result is 'OK', and logs/rethrows on error.
Builtin instance control lib/services/builtin_instance_service.dart	Replaces string interpolation with p.join for paths, delegates default download dir to utility, appends --save-session-interval=30 and --force-save=false to Aria2 args, adds public pid getter, and updates stopInstance() to cancel streams, attempt _shutdownThroughRpcIfPossible() (saveSession + RPC shutdown) with timeouts, then kill/wait as fallback.
Notification → persistence wiring lib/services/download_data_service.dart	_collectTaskNotifications now returns Set<String> of instance IDs that had terminal task transitions; refreshTasks captures those IDs and calls _saveSessionsForTerminalTransitions() to call client.saveSession() for affected connected instances.
Cosmetic UI lib/app.dart	Reformats a Text(...) call for the total-speed Chip to a single-line expression without changing behavior.

Possibly related PRs

• opt: Remove dead code #73 — Modifies similar service and settings code; potential overlap around pid getter and settings behavior.
• opt: Cleanup #74 — Also changes Aria2RpcClient and touched settings/app files; likely related to the new RPC shutdown API and settings adjustments.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately captures the two main changes: fixing the default download directory and persisting built-in aria2 sessions through RPC shutdown.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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[coderabbitai]

🧹 Nitpick comments (1)

lib/services/builtin_instance_service.dart (1)

106-124: 💤 Low value

Synchronous _defaultDownloadDir() duplicates logic from settings.dart.

This method mirrors _defaultDownloadDirectory() in lib/models/settings.dart but is synchronous (doesn't use path_provider). Consider extracting a shared utility to avoid drift between the two implementations.

🤖 Prompt for AI Agents

Verify each finding against the current code and only fix it if needed.

In `@lib/services/builtin_instance_service.dart` around lines 106 - 124, Duplicate
logic exists between the synchronous _defaultDownloadDir() and the async
_defaultDownloadDirectory() (which uses path_provider); extract the shared
decision logic into a single utility (e.g., getDefaultDownloadDirectorySync or a
sync helper) and have _defaultDownloadDir() call that helper while refactoring
the async _defaultDownloadDirectory() to reuse the same logic (or call a common
async wrapper that uses path_provider when available). Update references in
BuiltinInstanceService._defaultDownloadDir() and the settings model's
_defaultDownloadDirectory() to call the new shared utility so the behavior stays
consistent and avoids drift.

🤖 Prompt for all review comments with AI agents

Verify each finding against the current code and only fix it if needed.

Nitpick comments:
In `@lib/services/builtin_instance_service.dart`:
- Around line 106-124: Duplicate logic exists between the synchronous
_defaultDownloadDir() and the async _defaultDownloadDirectory() (which uses
path_provider); extract the shared decision logic into a single utility (e.g.,
getDefaultDownloadDirectorySync or a sync helper) and have _defaultDownloadDir()
call that helper while refactoring the async _defaultDownloadDirectory() to
reuse the same logic (or call a common async wrapper that uses path_provider
when available). Update references in
BuiltinInstanceService._defaultDownloadDir() and the settings model's
_defaultDownloadDirectory() to call the new shared utility so the behavior stays
consistent and avoids drift.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: 6df8c363-dce3-40d5-a612-4db79a1246d8

📥 Commits

Reviewing files that changed from the base of the PR and between 2111303 and cc47d3e.

📒 Files selected for processing (4)

• lib/models/settings.dart
• lib/services/aria2_rpc_client.dart
• lib/services/builtin_instance_service.dart
• lib/services/download_data_service.dart