feat: shell terminal, virtual keyboard fixes, settings font slider

- Add shell terminal: new tmux window spawned at project cwd, opened
  from the Terminal icon in the ChatView header; ShellTerminalView lets
  you navigate back to chat or jump to the Claude tmux terminal mode
- Fix system keyboard appearing on first terminal launch: ghostty-web
  calls focus() on a contenteditable div inside open(), triggering iOS
  keyboard before we can suppress it; patch HTMLElement.prototype.focus
  as a no-op for the synchronous duration of terminal.open()
- Add keyboard font-size slider in Settings
- Fix virtual keyboard button corner radius and depth scaling with key height
- Fix keybar gap/padding proportionality via heightScale factor
- Cap keybar font size so it does not grow when bar height is increased

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/lib/api.ts

@@ -170,4 +170,10 @@ export const api = {
 
   deleteInstruction: (id: string) =>
     request<void>(`/api/instructions/${id}`, { method: 'DELETE' }),
+
+  newShellTerminal: (cwd: string) =>
+    request<{ sessionId: string }>('/api/terminal/shell', {
+      method: 'POST',
+      body: JSON.stringify({ cwd }),
+    }),
 };