[mhrstmnn]

Summary

The web UI now follows the OS color scheme until the user picks a theme.

• ThemeContext keeps the OS scheme in state and updates it from a prefers-color-scheme change listener, so an open tab switches live.
• Only an explicit toggle writes the theme, under the new backlog-theme-choice key. Toggling back to the theme the OS shows removes the key, so the UI follows the OS again. The single toggle button is unchanged.
• The old backlog-theme key held a theme written on every load, not a real choice, so it is no longer read. Existing users follow the OS after upgrading.
• The pre-render script in index.html applies the same rule, so the page doesn't flash the wrong theme.

Related Issue or Task

Closes #1158, BACK-795

Task Checklist

• I have created a corresponding task in backlog/tasks/
• The task has clear acceptance criteria
• I have added an implementation plan to the task
• All acceptance criteria in the task are marked as completed

Testing

• New src/test/web-theme.test.tsx covers following OS changes, storing nothing on load, pinning, clearing the choice and ignoring the old key. All four cases failed before the change.
• bun run check:types, bun run check ., bun run build and all 387 web tests pass.
• Checked live in Brave 1.98.60 through Playwright: emulated OS changes switch the theme without a reload; a pinned theme survives OS changes and a reload; toggling back follows the OS again; an old backlog-theme=dark value is ignored.
• With the app bundle blocked, the index.html pre-render script alone picks the expected theme for each combination of stored choice and OS scheme.