feat(ui): selectable UI themes (10 palettes + swatch picker)

The UI was hardcoded to one Catppuccin Mocha palette (color literals in
view() + theme() pinned to Dark). It now sources colours from a chosen
theme's palette, selectable in Settings.

- theme.rs (NEW): Palette (13 semantic colour roles) + AppTheme enum
  (Catppuccin Mocha/Macchiato/Frappe/Latte, Dracula, Nord, Tokyo Night,
  Gruvbox Dark, Solarized Light, Gruvbox Light). Pure palette()/label()/
  ALL/base_theme()/is_dark() + WCAG relative_luminance()/contrast_ratio().
- config: theme: AppTheme field (serde-default Mocha) + backward-compat.
- app: theme() returns config.theme.base_theme() (iced widget chrome);
  view() + with_layout_picker() source colours from the palette; new
  ThemeSwatch canvas widget; a Theme section of clickable swatches in
  Settings; SelectTheme applies live + persists. Canvas widgets already
  take colours as data, so they re-theme for free.

Tests written alongside (+7 theme, +2 config; 135 -> 144 lib): every
palette clears WCAG AA text-on-base contrast (4.5:1) with subtext/accent
>= 3:1, is_dark matches luminance direction, variants distinct/labeled,
serde round-trips, default = Mocha.

Verified live: the Settings swatch grid renders all 10 palettes and the
whole UI re-themes (screenshot-checked Latte light + Dracula dark).
clippy --all-targets clean.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/FEATURES.md

@@ -115,6 +115,7 @@ covers internals). When you ship a feature, add it here.
 | Chat drawer toggle (drawer layout) | 🧪 | Toggle + restart-persistence not explicitly confirmed. |
 | Responsive full-window Settings screen | ✅ | |
 | Canvas-drawn icon set | ✅ | 18 icons (`Icon`/`IconKind` in `src/app/mod.rs`), no image/font dep, theme-recolored. Replaces all emoji; SVG design source in `~/Documents/peerspeak-mockups/icons/`. |
+| Selectable UI themes | ✅ | 10 palettes (`src/theme.rs`): Catppuccin Mocha (default)/Macchiato/Frappé/Latte, Dracula, Nord, Tokyo Night, Gruvbox Dark, Solarized Light, Gruvbox Light. Picked via canvas palette swatches in Settings (`ThemeSwatch`), applied live + persisted (`config.theme`). All palettes unit-tested for WCAG-AA text legibility. |
 
 ## Persistence