fix(settings): Change default arrangement setting with the in-song menu

[jphinspace]

In-song arrangement (lead, rhythm, bass, etc) changes were not being persisted to subsequent songs. This might have been intended but IMO the normal default arrangement setting is harder to find than the in-song-menu one, and I'm not sure there's much value in having two totally independent settings. Most of the time I'm not switching instrument between every song.

Summary by CodeRabbit

• Bug Fixes
  • Changing the song arrangement during playback now also saves that arrangement as the default automatically.
  • If saving fails, the arrangement switch still completes normally without interruption.
  • Added regression coverage to confirm the default arrangement indicator updates correctly after a manual switch.

[coderabbitai]

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 6b096933-1642-4c1e-8044-70bebc6c372b

📥 Commits

Reviewing files that changed from the base of the PR and between 950e348 and 4bf6bd9.

📒 Files selected for processing (2)

• static/app.js
• tests/browser/default-arrangement-pin.spec.ts

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📝 Walkthrough

Walkthrough

Manual arrangement switching now triggers an unawaited call to persist the selected arrangement as the default, swallowing any errors. A new Playwright regression test verifies that switching arrangement via the dropdown results in a settings POST with the correct default_arrangement and updates the pin UI state.

Changes

Auto-persist arrangement default

Layer / File(s)	Summary
Fire-and-forget persistence on manual arrangement switch static/app.js, tests/browser/default-arrangement-pin.spec.ts	changeArrangement now calls pinCurrentArrangementDefault() without awaiting, catching and swallowing failures; a new regression test intercepts /api/settings POSTs, mocks WebSocket, switches arrangement via the #arr-select dropdown, and asserts the persisted default_arrangement and updated #arr-default-pin state.

Estimated code review effort: 2 (Simple) | ~10 minutes

Sequence Diagram(s)

sequenceDiagram
  participant User
  participant AppJS as app.js
  participant SettingsAPI as /api/settings

  User->>AppJS: change `#arr-select` (dispatch change event)
  AppJS->>AppJS: changeArrangement(index)
  AppJS->>AppJS: pinCurrentArrangementDefault() (not awaited)
  AppJS->>SettingsAPI: POST default_arrangement
  SettingsAPI-->>AppJS: response (success or error)
  AppJS->>AppJS: swallow error if any
  AppJS->>User: update `#arr-default-pin` aria-pressed

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🚥 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 clearly summarizes the main change: in-song arrangement changes now update the default setting.
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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