chore(tsconfig): drop stale dashboard build comment and vestigial exclude

The exclude comment described src/dashboard as "the Svelte browser bundle —
compiled separately by scripts/build-dashboard-ui.mjs ... covered by
tsconfig.dashboard.json for editor + CI type-checking." None of that is true:

- src/dashboard/ is server-rendered HTML (htmx + Alpine, both inlined by
  scripts/vendor-ui.mjs) — not Svelte, and there is no esbuild-svelte step
- scripts/build-dashboard-ui.mjs and tsconfig.dashboard.json do not exist
- CI (.github/workflows/ci.yml) runs only the root typecheck/test/build

The exclude itself was a no-op: src/dashboard.ts (which IS in the program)
imports ./dashboard/fragments.js and ./dashboard/types.js, so tsc already
type-checks and emits dist/dashboard/* transitively. Verified: removing the
exclude leaves typecheck green and dist/dashboard/* emits identically with or
without it. Dropped both the false comment and the dead exclude entry.

tsconfig.json

@@ -25,12 +25,6 @@
   "exclude": [
     "node_modules",
     "dist",
-    "legacy",
-    // src/dashboard/ is the Svelte browser bundle — compiled separately by
-    // scripts/build-dashboard-ui.mjs with the DOM lib + esbuild-svelte. The
-    // root tsconfig targets Node + WebWorker (no `document` / `localStorage`),
-    // so type-checking dashboard sources here would always fail. They're still
-    // covered by `tsconfig.dashboard.json` for editor + CI type-checking.
-    "src/dashboard"
+    "legacy"
   ]
 }