feat(export): export transcripts to .txt and .vtt (#441)

[tayyub-ai]

Description

Adds an Export dropdown to the meeting transcript toolbar with two options: plain text (.txt) and WebVTT (.vtt). Both formats are built in Rust (unit-tested) and written to a user-chosen path via the existing tauri-plugin-dialog save dialog. Disabled while recording or when no transcripts exist.

.txt format

[00:01:23] We should ship the feature.
[00:01:30] Agreed.

One line per segment, sorted by audio_start_time. Timestamp bracket omitted when the segment has no timing (e.g. imported audio). Internal newlines collapsed.

.vtt format

Standard WebVTT v1 with &, <, > escaping. End-time fallback chain: audio_end_time → audio_start_time + duration → audio_start_time + 5.0. Segments without a start time are skipped and the count is surfaced in a sonner warning toast. Overlapping mic/system audio segments are kept as distinct cues.

Default filename

<sanitized meeting title> - <YYYY-MM-DD>.{txt|vtt} (date from meeting.created_at).

Related Issue

Fixes #441

Type of Change

• New feature

Testing

• 20 Rust unit tests added in frontend/src-tauri/src/api/export.rs:
  • format_txt (7): happy path, NULL timestamp, newline collapse, ordering, hour rollover, empty input, trailing newline invariant
  • format_vtt (8): happy path, duration fallback, 5s fallback, NULL-start skipping, character escaping, overlapping cues, hour-boundary formatting, empty input
  • sanitize_filename_stem (5): banned characters, whitespace collapse, empty fallback, whitespace-only fallback, 80-char truncation
• pnpm run build passes (TypeScript + Next.js static generation)
• Fork CI green on macOS + Linux 22.04 + Linux 24.04 (see notes on Windows below)
• Manual VLC verification not performed by contributor — my dev environment lacks a full local Tauri build setup; would appreciate a maintainer with a configured macOS dev env loading an exported .vtt into VLC against a real recording to confirm timing sync.

Documentation

• No user-facing docs change needed (button is self-explanatory)

Notes for reviewers

Windows CI failure is pre-existing and unrelated. The Windows job fails inside llama-cpp-sys-2 v0.1.146's build script when CMake tries to build the bundled vulkan-shaders-gen tool (Not a file: cmake_install.cmake). This is in the llama-helper sidecar's dependency chain — a path my changes don't touch. Evidence:

• The same failure occurred on this PR's first CI run before any of my Rust changes had even been compiled (at a step that runs before the Tauri build).
• Open PR fix: upgrade whisper-rs to 0.16.0 and fix Windows build #460 ("upgrade whisper-rs to 0.16.0 and fix Windows build") is the upstream effort to address Windows build chain issues. Happy to rebase once that lands.

Speaker labels were intentionally excluded from v1. The transcripts.speaker column exists from migration 20251110000001_add_speaker_field.sql, but no writer in the live code populates it. Emitting <v Speaker> cues today would only ever produce blanks. Deferred to a follow-up paired with whoever wires diarization end-to-end.

pnpm-lock.yaml diff includes a small overrides reconciliation. pnpm install --frozen-lockfile was already failing against the pre-existing lockfile on devtest (overrides-config mismatch). The non-frozen install fixed that drift in the same hunk as the new @tauri-apps/plugin-dialog dep. Happy to split into a separate chore(deps) commit if you'd prefer.

dialog:allow-save is the only new capability. The Rust tauri-plugin-dialog was already in Cargo.toml (2.7.1 per Cargo.lock) and registered in lib.rs:411. The JS package was pinned to match (^2.7.0) — see commit fix(export): bump @tauri-apps/plugin-dialog to ^2.7.0 for the version-mismatch fix discovered in CI.

Checklist

• Code follows project style
• Self-reviewed the code
• Branch targets devtest
• No unrelated changes (lockfile reconciliation noted above)