refactor: take the theme, palettes and filesystem layout from colony-ui

[MotherSphere]

Completes the migration merged in #76, and carries the changelog entry it should have had.

#76 was merged with a merge commit. release-please reads the merge commit’s own message — Merge pull request #76 from … — not the conventional commits inside the branch, so it opened no release PR. A 3604-line change that moves users’ config and cache directories landed on main with nothing in the changelog. Squashing this PR fixes that.

The content

Two contributor docs the migration left stale:

• CONTRIBUTING.md still said src/ui/theme.rs holds all 25 families and 57 palettes. It is 103 lines of re-export now.
• It also listed “new theme” as a small feature to PR here. Adding a theme family no longer touches this repository at all — the TOML goes upstream, and Colony picks it up on the next colony-ui bump.

What #76 did

• src/ui/theme.rs: 2935 lines to 103. The palettes come from colony-ui, generated from shared design tokens.
• The theme picker renders from the generated catalog instead of a 367-line hand-kept list.
• Config, data and cache follow the shared Colony/<Program>/ layout. Existing files migrate on first start without the originals ever being deleted. On Windows the config root moves from Roaming to Local, where it always should have been.
• Caches, cached docs and icons, and update staging move out of the config directory into the cache root.

150 tests, green on Linux, macOS and Windows.