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/actions.ts

@@ -0,0 +1,23 @@
+// Canonical action + writing-style lists shared by the toolbar, popup,
+// context menus, and result modal. Values must stay in sync with the system
+// prompts in the background worker.
+
+// 'fix' is the user-facing id emitted by the context menu and popup; the
+// background normalizes it to the 'grammar' prompt.
+export const ACTIONS = ['fix', 'rephrase', 'shorten', 'expand', 'explain'] as const;
+export type ActionId = (typeof ACTIONS)[number];
+
+export const ACTION_LABELS: Record<ActionId, string> = {
+  fix: 'Fix Grammar',
+  rephrase: 'Rephrase',
+  shorten: 'Shorten',
+  expand: 'Expand',
+  explain: 'Explain',
+};
+
+// Full list, including the 'Default' pseudo-style (= no style modifier).
+export const WRITING_STYLES = ['Default', 'Formal', 'Casual', 'Academic', 'Creative', 'Concise'] as const;
+export type WritingStyle = (typeof WRITING_STYLES)[number];
+
+// Context menus omit 'Default' — the parent action item already covers it.
+export const CONTEXT_MENU_STYLES = WRITING_STYLES.filter((s) => s !== 'Default');