Allow desktop theme managers to apply a theme file

[PintaAI]

Desktop theme managers can change the rest of a user's environment, but T3 Code currently requires a manual theme import and selection each time.

I propose a small desktop-only --theme-file=<path> interface that:

• accepts the same T3 Code ThemeFile v1 and VS Code color-theme JSON already supported by the Appearance import flow;
• imports and activates the theme at startup or forwards it to an already-running desktop instance;
• updates an installed theme with the same ID instead of creating duplicates; and
• retains the existing 256 KiB theme-file limit.

This keeps integrations outside T3 Code. For example, an Omarchy theme-set hook can pass its generated ~/.local/state/omarchy/current/theme/vscode-theme.json; other desktop theme managers can generate either supported format.

I tested the implementation against the v0.0.33 release source on Omarchy, including the real Electron second-instance handoff, and confirmed the generated theme was activated and persisted. The focused tests, web/desktop typechecks, lint, and production build also pass on current main.