feat: wire theme provider and appearance picker

[calebmcquaid]

Context & Requests for Reviewers

This PR adds a ThemeProvider that defaults to light. ThemeSync applies the users saved choice. The users choice is setup in #1220. This also adds a theme picker in Account Settings.

Tests

Clean build, tests pass. No tests added for this.

Checklist

Only check items that apply to this PR; leave the rest unchecked.

• If you changed anything user-facing (i.e. user interface or APIs):
  Did you update related docs?

• If the change is notable (refer to Keep a Changelog conventions):
  Did you update CHANGELOG.md?

• If you changed db/src/scripts/** and used CREATE TABLE, ADD COLUMN, or ALTER COLUMN:
  Are as many columns marked NOT NULL as possible? If some columns can sometimes be null depending on other columns, are there CHECK constraints capturing those relationships, and are these also reflected using unions in the associated Kysely types?

• If you added a new signal in server/services/signalsService/signals/**:
  Did you classify every error case as a permanent error (SignalPermanentError, no retry) or a normal error (retryable)? Any case where the signal can't determine a score should be a SignalPermanentError.

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Summary by cubic

Adds a theme picker to Account Settings. Users can now choose Light, Dark, or System, and the selection persists across sessions and applies on login. The default stays light until the app-wide dark restyle lands.

• Saves the preference server-side via a new setThemePreference mutation.
• Applies the persisted theme using next-themes and the new ThemeSync component.
• Updates dashboard and app backgrounds to use theme tokens instead of hardcoded colors.

^{Written for commit 1aa326c. Summary will update on new commits.}

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