fix(browser): forward editing and navigation keys through the remote keyboard

[devswha]

Closes #20.

What broke

Printable characters worked in the Browser panel's remote inputs, but Backspace/arrows/Home/End/Tab and modifier combinations did nothing. Two causes:

1. Key bridge — browser-sidecar.ts dispatched raw CDP Input.dispatchKeyEvent with only { type, key, code, modifiers }. Without windowsVirtualKeyCode/nativeVirtualKeyCode, Chromium delivers a DOM key event but never runs its default editing command.
2. Focus — keyboard handlers live on the tabIndex=0 surface, but pointer events target the child <img>, which never focused the surface. Keys could keep going to the address bar or chat input after clicking into the page.

Fix

• Recognized keys (editing/navigation keys, modifiers, F1–F12, printable) now go through Puppeteer's page.keyboard.down/up — the same path the automation commands already use — via a pure, unit-tested toPuppeteerKeyInput mapping (OS→Meta, Esc→Escape, etc.). Puppeteer tracks modifier state from the forwarded Shift/Control/Alt/Meta events, so Cmd/Ctrl+A/C/V and Shift+Arrow selection work.
• Keys with no KeyInput mapping (IME/dead keys) keep the raw CDP dispatch as fallback.
• Pointer down on the streamed frame focuses the keyboard surface; composing keydown events (IME) are left alone.
• The sidecar entrypoint is now guarded by a main-module check so the mapping helper is importable from unit tests.

Coverage

• New browser-sidecar-keys.test.ts: editing/navigation keys, aliases, printable characters, F1–F12, rejected IME keys.
• Full server/modules/automation suite: 25/25 pass (includes the sidecar restart/protocol tests).
• Both tsconfigs typecheck clean; eslint clean.

The end-to-end matrix from the issue (type abcd, ArrowLeft×2, Backspace, Delete; Home/End; Shift+Arrow; Tab/Shift+Tab; Enter/Escape; Cmd/Ctrl+A,C,V) is exercised by the Puppeteer path the automation commands already use; CUA-level verification is left to the next sandbox pass.