feat: rebuild Preferences, typography and sidebar navigation on the shared colony-ui kit

[MotherSphere]

Why

Colony's Preferences page, fonts, text sizing and sidebar were the hand-written originals that colony-ui 0.1.6 now ships as a shared kit. Keeping both meant two copies drifting apart, and a few defects only Colony's copy had.

Changes

Preferences

• colony-ui 0.1.6 (iced stays at 0.14.0). The ten Appearance and Accessibility preferences are its StandardPreferences, flattened into preferences.json under the field names the file already used, applied at boot and changed through a single Preference(Change) message.
• A file written by 0.10.4 or earlier stored "selected_accent": "blue" for everyone who never picked an accent, and applied it over the theme's own. It becomes the theme's own accent, once: a new prefs_version field keeps a blue picked later. Values this build does not understand fall back to their defaults and the rest of the file is kept.
• The page is built from the kit: General, Appearance, Accessibility, then Storage, Shortcuts and About last. Appearance and Accessibility come from PreferencesView, with Colony's keyboard navigation between Motion and Reading. Colony's own sections use the kit's collapsible sections, rows, dropdowns and toggles, and fold again when the page closes. The hand-written theme picker, the accent hex table and the white check marks are gone.
• Locale keys the kit provides are removed from en.rs and fr.rs.

Typography, motion, navigation

• The fonts ship inside colony-ui: src/ui/assets/fonts/ is removed and the README links the licences in colony-ui.
• Every text size goes through colony-ui's sz(), every font through its accessors (no synthesized weights). Nerd Font glyphs are drawn in the Nerd Font, apart from their labels, so they survive the dyslexia font. Markdown table headers, cells and badges follow the text scale; badge text is contrast_on the badge colour.
• Every animation is gated on effects_enabled().
• The sidebar uses identity_button, nav_label, nav_item styling and nav_hint: selection by accent background with contrast_on text. Its width, like the category list's, grows with the text scale.
• categories.json writes its glyphs as \uXXXX escapes, with a test. Light/Dark comes from the palette's own lightness rather than a local luma formula.

French interface

• The English words left in French (Rescan, Scan, License, Changelog and others) are translated, and descriptions that promised settings Colony does not have (time format, update channel, scan directories, private repository access) are corrected in both locales.
• The 21 merged PRs whose descriptions were in French now read in English.

Layout

• validate-manifest moves to manifest.rs (output and exit codes unchanged, now under test) and the logging setup to logging.rs. main.rs keeps argument dispatch and the launch.
• download.rs (2068 lines) becomes download/ with http.rs, files.rs, verify.rs, install.rs and launcher.rs, a pure move with the tests next to their code.
• The shields.io colour names move to config/shields-colors.json; a colour string with a multi-byte character no longer panics the parser.
• docs/architecture.md, docs/tutorial.md, docs/faq.md, CONTRIBUTING.md and docs/release-signing.md follow.

Testing

• cargo test --bin colony: 164 passed locally, including the new 0.10.4 preferences fixture, the accent migration, the category order, key parity, escaped glyphs, shields colours and validate-manifest exit codes.
• CI covers Windows, macOS, clippy and the MSRV.