ui: add custom stepper controls for numeric settings

[bashrusakh]

Closes #57

Summary

Replace standard browser number inputs with custom stepper controls featuring +/- buttons in Settings → Transfer section.

Problem

Numeric fields (Connections per file, Concurrent downloads, Retry attempts) used white browser spinners that didn't match the app's dark theme. The "Verification mode" dropdown was styled correctly and served as the reference.

Changes

HTML (index.html):

• Wrap connections, maxActive, retries fields in .stepper-control
• Add - and + buttons with proper ARIA labels
• Add data-default attributes for value restoration

CSS (style.css):

• Add .stepper-control flex layout
• Style input field (dark background, centered text, no native spinners)
• Style stepper buttons with hover/active states matching theme
• Purple accent on active state (like Verification dropdown focus)
• Disabled state at min/max boundaries

JavaScript (app.js):

• Add initStepperControls() function
• Proper NaN handling with parseBound() helper
• Clamp values on blur, not during typing
• Button state updates on value changes

Screenshots

Before: White browser spinners
After: Styled +/- buttons matching dark theme

Testing

• Click +/- changes value by 1
• Direct input works
• Min/max boundaries respected
• Buttons disabled at boundaries
• Values clamp on blur
• Empty values restore default on blur
• Settings save correctly

Checklist

• Code follows project style
• Tests pass (go test ./internal/server)
• JS syntax valid (node --check)
• Code review issues addressed

Summary by CodeRabbit

• New Features / UI Improvements
  • Updated numeric settings fields (e.g., transfer connections, max active, and retries) to use stepper controls with “−” and “+” buttons.
  • Added refined stepper styling, including hover/active/disabled visual states and cleaner number-input presentation.
  • Improved number input behavior: enforces min/max limits, refreshes button enablement while typing, and restores/clamps values on blur when empty or invalid.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: e152d950-97f7-4e72-9246-ae42d6b0429c

📥 Commits

Reviewing files that changed from the base of the PR and between aab8fcf and 3cf4b1d.

📒 Files selected for processing (1)

• internal/assets/static/index.html

🚧 Files skipped from review as they are similar to previous changes (1)

• internal/assets/static/index.html

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📝 Walkthrough

Walkthrough

Adds stepper controls to the Transfer settings numeric fields. The settings UI now wraps connections, maxActive, and retries in stepper-control containers with −/+ buttons. CSS and JavaScript styles and wire the controls, enforce bounds, and restore valid values on blur.

Changes

Stepper Control for Transfer Settings

Layer / File(s)	Summary
CSS and stepper logic internal/assets/static/css/style.css, internal/assets/static/js/app.js	Adds .stepper-control styling and initializes stepper behavior for number inputs, including button state updates, min/max enforcement, and blur handling.
Transfer settings markup internal/assets/static/index.html	Wraps the connections, maxActive, and retries number inputs in .stepper-control containers with data-action buttons and data-default values.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly summarizes the main UI change: custom stepper controls for numeric settings.
Linked Issues check	✅ Passed	The PR implements the requested stepper controls, styling, and validation for all three Transfer numeric fields.
Out of Scope Changes check	✅ Passed	The changes stay focused on the requested Settings Transfer numeric input redesign and its supporting behavior.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch settings-stepper-control

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[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@internal/assets/static/index.html`:
- Around line 741-743: Update the stepper controls so each button in the related
input groups uses a field-specific aria-label instead of the generic
“Decrease”/“Increase”; adjust the buttons associated with the connections,
timeout, and the third stepped setting to include the setting name in their
labels. While updating the button markup, add aria-controls on each stepper
button to point to its corresponding input element, using the existing stepper
button structure and input ids to locate the controls.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: 221f0e4b-a31f-4895-980c-4c53d825bc82

📥 Commits

Reviewing files that changed from the base of the PR and between 91d36b6 and aab8fcf.

📒 Files selected for processing (3)

• internal/assets/static/css/style.css
• internal/assets/static/index.html
• internal/assets/static/js/app.js