feat(ui): design-system foundation — shared token layer + fonts

Establish the visual-redesign foundation (task group 1): a single
shared source of design tokens both apps consume, so Planner and
Journal read from one palette/typography layer instead of drifting
per-app.

- New @trails-cool/ui package exporting theme.css: Tailwind v4 @theme
  tokens (warm off-white surfaces, one sage accent, earthy overnight
  tones, elevation-gradient colors, shadows) extracted from the
  visual-redesign mockup.
- Self-hosted Outfit (body) + Geist Mono (stats) via @fontsource-variable
  — privacy-first, no Google Fonts CDN.
- Both apps import the theme and set the base canvas to the warm
  off-white token + Outfit as the default font.

Foundation only: tokens are now available as Tailwind utilities
(bg-bg-raised, text-accent, font-mono, …) and raw CSS vars. Wiring
them into specific surfaces (topbar, sidebar, markers, elevation
chart) is later task groups.

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

openspec/changes/visual-redesign/tasks.md

@@ -1,9 +1,9 @@
 ## 1. Design Tokens & Typography
 
-- [ ] 1.1 Add CSS custom properties (`:root` vars) for colors, shadows, borders
-- [ ] 1.2 Extend Tailwind config with project color tokens and font families
-- [ ] 1.3 Add Outfit + Geist Mono fonts (Google Fonts or self-hosted)
-- [ ] 1.4 Update base styles: body background, default text color, font-family
+- [x] 1.1 Add CSS custom properties (`:root` vars) for colors, shadows, borders
+- [x] 1.2 Extend Tailwind config with project color tokens and font families
+- [x] 1.3 Add Outfit + Geist Mono fonts (Google Fonts or self-hosted)
+- [x] 1.4 Update base styles: body background, default text color, font-family
 - [ ] 1.5 Update elevation gradient colors in ColoredRoute + ElevationChart to use tokens
 
 ## 2. Topbar Redesign