feat(theme) user theme auto-reload

[mroemore]

Issue for this PR

Closes #14847

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Custom themes in the XDG config directory (~/.config/opencode/themes/) now reload automatically when edited. Previously you had to restart the TUI or send a SIGUSR2 signal to pick up changes.

I added a @parcel/watcher subscription inside the ThemeProvider that watches the themes directory. When a .json file is created, modified, or deleted, it re-reads the custom themes and syncs them to the store. There's a 300ms debounce to handle atomic writes. editors like Vim and VS Code write to a temp file then rename, which fires multiple watcher events.

Only the XDG config directory is watched, since that's where users iterate on personal themes. Project-level .opencode/themes/ directories are typically set once and rarely changed. Could add a setting to disable it but I don't see much reason.

How did you verify your code works?

Wrote 5 unit tests covering: new theme file added, existing theme modified, theme file deleted, missing directory handling, non-JSON file ignored. I of course manually tested in the TUI. I edited a theme JSON file and confirmed the changes appeared within approx 300ms consistently. It passes all type checking, too.

Screenshots / recordings

N/A — no visual changes, just background file watching behavior.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

Hey! Your PR title Feat/theme auto reload doesn't follow conventional commit format.

Please update it to start with one of:

• feat: or feat(scope): new feature
• fix: or fix(scope): bug fix
• docs: or docs(scope): documentation changes
• chore: or chore(scope): maintenance tasks
• refactor: or refactor(scope): code refactoring
• test: or test(scope): adding or updating tests

Where scope is the package name (e.g., app, desktop, opencode).

See CONTRIBUTING.md for details.