Fix global shortcut PiP when no active tab

[jotenbai]

This PR fixes a bug where the global shortcut for the PiP command sometimes fails to start Picture-in-Picture when Chrome does not provide an active tab to the chrome.commands handler.

Previously, getAutoMedia would return early when tabInfo was undefined, so in those cases no media element was selected and the PiP command did nothing. Now, when tabInfo is missing we still score all known media items and pick the best candidate, while continuing to respect pinned media and existing PiP when available.

Testing:

• Built the extension (NODE_ENV=development npm run build:common) and loaded it unpacked in Chrome.
• Configured command A as a global shortcut mapped to PiP with value state toggle.
• Verified that:
  • Global shortcut can toggle PiP off reliably in foreground and background.
  • Starting PiP via the global shortcut works more reliably than before even when there is no active tab passed to the command handler, and no longer fails just because tabInfo is missing.