refactor: extract Settings UI into src/settings

[enaboapps]

Closes #683.

Moves the Settings UI out of App.tsx into src/settings/ as a pure mechanical refactor, so the tabbed-sections restructure in #685 lands with a readable diff.

Moved verbatim

From App.tsx	To
SettingsView	src/settings/SettingsView.tsx
SettingGroup, Toggle, option constants, movementValue	src/settings/controls.tsx
UpdateControls, updateProgress, updateDescription, UpdateAction	src/settings/UpdatesSection.tsx
changedSettingKeys, applyLocalSettings	src/settings/diff.ts

UpdateBanner stays in App.tsx as shell-level UI and imports updateProgress back from the new module. The autosave state machine stays in App.tsx in full — only the two pure helpers moved.

App.tsx: 838 → 684 lines. No DOM changes. No AppSettings changes.

Tests

The 19 Settings tests move to src/settings/settings.test.tsx with no assertion changes. They still render <App />, so the full save pipeline stays under test.

The two banner-level updater it.each blocks stay in App.test.tsx — UpdateBanner did not move, and they assert banner behavior rather than Settings content.

Test titles were diffed against main as a set: identical, so nothing was dropped in the move.

Validation

Run from the repository root:

Check	Result
npm run lint	pass
npm test	pass — 75 tests, 4 files (settings.test.tsx 19, App.test.tsx 45, plus api/overlay)
npm run build	pass
cargo fmt --check	pass
cargo clippy --all-targets -- -D warnings	pass (exit 0)
cargo test	pass (exit 0) — 263 + 7 tests

No Rust files are touched by this change; the Rust gates are run per AGENTS.md.

Notes for review

• The plan estimated App.tsx would land near 600 lines; the actual is 684.
• Per-section components (GeneralSection.tsx, PointerSection.tsx, …) are not created here. Those sections do not exist as separate components yet, so creating them would have meant editing behavior rather than moving it. They arrive with Restructure Settings into accessible tabbed sections #685.
• Ref was dropped from the App.tsx React type import and AppSettings from App.test.tsx, both now unused there.

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