fix(settings): persist the "System" theme choice

[Adron]

Summary

Closes #79. Settings → Appearance → Theme → System now sticks.

The picker mapped "system" to nil, and Swift's synthesized Encodable omits a nil field
(encodeIfPresent), so the PATCH /api/user/update body encoded to {}. The route applies
...(theme !== undefined && { theme }), so an absent key is a no-op: the stored theme was never
changed, the response returned the unchanged user, and syncFromUser snapped the picker back to
the old value.

The comment justifying the nil mapping — "Sending the string "system" is rejected or treated as
default (light) by the server" — was factually wrong. The route destructures theme and stores
whatever string arrives with no validation (app/api/user/update/route.ts:24,102,157), and the web
persists the same literal "system" (components/layout/ThemeProvider.tsx:11,17), so the value
round-trips with the web Settings page.

What's included

• InterlinedList/Views/SettingsView.swift — send newValue verbatim from the theme picker;
  delete the nil mapping and the incorrect comment. The spurious-save guard is unchanged.
• InterlinedListTests/APIClientTests/APIClientProfileTests.swift — two tests pinning the wire
  format: theme: "system" sends exactly {"theme":"system"}, and a nil theme omits the key
  entirely (the silent no-op this bug rode on, with the route semantics noted as the "why").
• RootView.preferredScheme verified, not changed: it returns nil for any non-light/dark
  value, so "system" already means "follow the OS".

No new files, so no project.pbxproj change.

Testing

xcodebuild -scheme InterlinedList \
  -destination 'platform=iOS Simulator,id=302E002E-9A0C-4F79-B54A-E9739A3EE582' \
  -derivedDataPath <worktree DerivedData> \
  -parallel-testing-enabled NO \
  -skip-testing:InterlinedListTests/E2EReadOnlyTests test

** TEST SUCCEEDED ** — Executed 1149 tests, with 0 failures, including the two new ones.

The encoder behaviour was also confirmed directly against the updateUserSettings body struct:
the old "system" → nil mapping encodes to {}, the fix encodes to {"theme":"system"}.

Not verified here (needs a live account): the round-trip through GET /api/user and an app
relaunch, and parity with the web Settings page.

🤖 Generated with Claude Code