feat(theme): add in-memory theme set registration

[BumpyClock]

Summary

Adds ThemeRegistry::register_theme_set(&mut self, set: ThemeSet), letting callers hand a ThemeSet straight to the registry without writing it to themes_dir first.

gpui-component-app's ThemeSource::Bundled currently has to sync embedded themes out to config_dir because ThemeRegistry's only ingestion path is reading files from disk. This adds a direct path, which also unblocks a future wasm build with no filesystem.

Behavior

• Registering a set overwrites any previous set with the same name (in-memory or previously loaded from disk).
• Default themes are protected the same way reload() protects them: a registered theme can't replace a built-in theme of the same name.
• If watch_dir later reloads a disk file with the same set name, the on-disk version wins on the next reload — registrations behave like a previously loaded file that disk can supersede.
• global_mut on this fork notifies observers, so callers just use ThemeRegistry::global_mut(cx).register_theme_set(set).

reload() is refactored to share its merge logic with the new method via a rebuild_theme_sets helper, caching the last disk-loaded sets so registration doesn't need to touch the filesystem.

Test plan

• cargo test -p gpui-component (186 passed, incl. 4 new registry tests)
• cargo clippy -p gpui-component --all-targets -- --deny warnings
• cargo fmt --check crates/ui/src/theme/registry.rs

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Review details

⚙️ Run configuration

Configuration used: Organization UI

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Run ID: 5ae4a2c2-0a27-4d1d-8a9b-b3e95c5b51e8

📥 Commits

Reviewing files that changed from the base of the PR and between 8be1306 and 97cb5b8.

📒 Files selected for processing (1)

• crates/ui/src/theme/registry.rs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/theme-registry-api

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