README: generate shortcuts table from shortcuts.ts (single source of truth)

The shortcuts table in README was hand-maintained and kept drifting from
src/lib/shortcuts.ts (the data the in-app help overlay reads). Replace the
table with a marker block (<!-- SHORTCUTS:START --> ... <!-- SHORTCUTS:END -->)
populated by scripts/gen-readme-shortcuts.mjs. Includes TIPS too, not just
shortcuts. Script is plain Node + fs (no tsx/esbuild dep); reads shortcuts.ts
as text, strips TS type syntax, dynamic-imports the resulting .mjs.

Adds `pnpm gen:readme` script and a `--check` mode that exits 1 on drift
(for future CI wiring). Idempotent.

package.json

@@ -10,6 +10,7 @@
     "test": "vitest run",
     "test:watch": "vitest",
     "check": "tsc --noEmit",
+    "gen:readme": "node scripts/gen-readme-shortcuts.mjs",
     "tauri": "tauri"
   },
   "dependencies": {