Add keyboard shortcuts

[DevOlabode]

Current behavior

There are no keyboard shortcuts in the extension today - no commands key in extension/manifest.json, and no keydown handling in extension/popup/popup.js beyond the browser's native form/button behavior.

Goal

Investigate and propose (this can start as discussion, then become a small focused PR) a small set of shortcuts that fit the popup's actual UI, for example:

• A toolbar command (via manifest.json commands, e.g. Ctrl+Shift+B) to open the popup directly into "Add a bookmark," prefilling the URL of the active tab if reasonable
• Esc to close the open composer (addForm / collectionAddForm in popup.js) instead of only the explicit Cancel button
• Esc to go back from a collection detail view or settings (#collection-back, #settings-back) to the library list

Constraints

• Don't introduce a shortcut that collides with common browser chrome shortcuts (e.g. Ctrl+T, Ctrl+W, Ctrl+L) or with shortcuts Chrome/Edge reserve for extension commands generally.
• Any new commands entry needs a sensible default that a user can still remap or disable from chrome://extensions/shortcuts.
• Keep it additive - existing mouse/touch interactions must keep working exactly as they do now.

Relevant files

• extension/manifest.json (commands)
• extension/background/service-worker.js (would need to listen for chrome.commands.onCommand)
• extension/popup/popup.js (composer open/close, back navigation)

Acceptance criteria

• Proposed shortcuts are listed with justification before implementation
• No collision with reserved browser shortcuts
• Existing navigation still works without a keyboard shortcut
• Shortcuts are discoverable (e.g. mentioned in extension/README.md or the options page)