refactor: single source for writing styles + action labels; add theme module

- src/lib/actions.ts: canonical ACTIONS/ACTION_LABELS/WRITING_STYLES;
  context-menu styles derived (WRITING_STYLES minus Default). Replaces
  the three duplicated style arrays in popup, content, and background.
- src/lib/theme.ts: canonical palette. Existing hex literals in the
  entrypoints are swapped wholesale when light/dark theming lands, so
  the color plumbing is only rewritten once.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/lib/theme.ts

@@ -0,0 +1,32 @@
+// Canonical LexAI palette (Catppuccin Mocha-derived). This is the single
+// source of truth for UI colors; a light variant can be added here later.
+//
+// NOTE: the entrypoints still contain literal hex values from before this
+// module existed. New/edited styles should reference THEME; the wholesale
+// swap of existing literals happens together with light/dark theming so the
+// color plumbing is only rewritten once.
+
+export const THEME = {
+  // Surfaces
+  base: '#1e1e2e',
+  mantle: '#181825',
+  surface: '#313244',
+  surfaceGradient: 'linear-gradient(135deg, #1e1e2e 0%, #181825 100%)',
+
+  // Text
+  text: '#cdd6f4',
+  subtext: '#a6adc8',
+  muted: '#6c7086',
+
+  // Accents
+  accent: '#89b4fa', // blue — primary actions, links
+  success: '#a6e3a1', // green — success toasts
+  danger: '#f38ba8', // red — errors
+  warning: '#f9e2af', // yellow
+
+  // Borders
+  border: 'rgba(205,214,244,0.12)',
+  borderStrong: 'rgba(205,214,244,0.15)',
+} as const;
+
+export type Theme = typeof THEME;