fix: stop the shell behaving like a browser

[omartelo]

The --app window is a browser only by construction, and its reflexes leaked
into every screen that is not a focused terminal — a terminal already swallows
them through xterm's own preventDefault, which is why they only ever misfired
elsewhere.

Two of them were fatal:

• Ctrl+W closed the window, and a closed window quits lich.
• Dropping a file on the window navigated to it, replacing the app with the
  file's contents, with no address bar to come back from. Easy to trigger by
  dragging a file at a terminal expecting to paste its path.

The rest were noise with real teeth:

Chord / gesture	What leaked
Ctrl+T, Ctrl+N (+Shift)	tabs and browser windows
Ctrl+P, Ctrl+S, Ctrl+O	print/save/open dialogs; Open navigates away and kills the app
Ctrl+U, Ctrl+H, Ctrl+J	view-source, history, downloads
Ctrl+Shift+Delete	wipes lich's own lich.* localStorage settings with the browsing data
Ctrl+Shift+M	profile switcher
Right-click on the UI	Back, Reload, Save as, Print, View source
Middle-click a link	browser window (onClick never sees an aux click)
Startup with a mismatched locale	the translate bubble, offering to translate lich's own UI

What changed

• internal/chromium: launch with --disable-features=Translate.
• frontend/src/lib/browser-defaults.ts (new): the chord table, the context-menu
  rule and the drop guard, installed from main.tsx before React mounts.
• Markdown.tsx: onAuxClick refuses the middle-click navigation the existing
  onClick handler could not see.

Swallowing is preventDefault and never stopPropagation: a terminal must
still receive every chord as its PTY sequence, since most of these are real
terminal bindings — Ctrl+W kills a word, Ctrl+U a line, Ctrl+D is end-of-file,
Ctrl+P walks shell history.

Kept on purpose

Reload (Ctrl+R / F5) recovers a wedged UI and costs nothing, since the session
lives in the backend; the devtools chords and F11 are worth more than the
purism; and the terminal keeps Chromium's right-click menu, which is where its
Copy and Paste live. Ctrl+F outside a terminal still opens Chromium's find bar —
searching the page is useful, and it is easy to add to the table later.

Test plan

• gofmt -l ., go vet ./..., go test ./... — 400 pass
• pnpm test — 416 pass, incl. the new browser-defaults table
• pnpm build, pnpm check
• Hand-checked in task dev. Chromium marks some accelerators
  reserved, and a reserved key cannot be cancelled by the page — Ctrl+W was
  the open question. It is not reserved in an --app window: the chord is
  refused and the window stays open.
• A terminal still gets Ctrl+W / Ctrl+U / Ctrl+D / Ctrl+P as PTY sequences
• Dragging a file onto the window leaves the app in place