chore: move biome config to biome.jsonc and baseline existing violations

biome.json is strict JSON; the jsonc extension allows the annotations
on the baseline entries. Rules that currently fail are downgraded to
warn with a note against issue #421 — restore each to error as its
occurrences are fixed. files.experimentalScannerIgnores keeps the
scanner out of .worktrees/ so a local worktree's own config can't
conflict with the root one.

Also fixes the handful of auto-fixable errors (unused imports in docs/
and landing/, @ts-expect-error over @ts-ignore). bun run lint is now
green: 0 errors, 96 warnings visible as debt.

app/index.html

@@ -6,7 +6,7 @@
     <meta name="viewport" content="width=device-width, initial-scale=1.0" />
     <title>voicebox</title>
     <script>
-      (function () {
+      (() => {
         try {
           var theme = 'system';
           var raw = localStorage.getItem('voicebox-ui');