fix: persist dark/light theme preference per user (#69)

[thermcampos]

Closes #69.

Summary

• The theme toggle was stored only in browser localStorage, so the preference was lost across browsers/devices and could drift after reloads. The choice is now persisted on the user record: a new nullable theme column (NULL treated as light), exposed on GET /rest/users/me and writable via PATCH /rest/users with strict light/dark validation (invalid values get a 400).
• The client keeps localStorage as a fast local cache so the theme paints immediately, then syncs from the server on login, session restore, and periodic refresh. A pending marker retries failed saves silently on the next sync instead of reverting the UI; signed-out behavior stays local-only, and logout leaves the theme untouched.
• Also fixes the browser router being recreated on every App render, which remounted the whole route tree (losing scroll position and unsaved form input) on each theme toggle.

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