feat(chat): drop vault notes and folders into the composer

[luoxuanzao]

Summary

• Add VaultDropController (src/features/chat/ui/vault-drop.ts): dragging notes/folders from the Obsidian file explorer onto the chat composer inserts @path / @path/ context mentions at the caret, skipping duplicates and showing a drop overlay while dragging. Borrowed from Claudian-plus M1 (drag notes and folders into chat).
• Wired in tab.ts before ImageContextManager so vault drags are claimed via stopImmediatePropagation without disturbing OS image drag/drop; destroyed in tab-lifecycle.ts.
• Only markdown files and non-root folders are accepted (matches the mention resolver's semantics); an input event is dispatched after insertion so send availability updates.
• i18n: new chat.drop.context key in all 10 locales; overlay CSS follows the existing image-drop visual language.

Verification

• npm run typecheck
• npm run lint
• npm run test (3361 passed, incl. 12 new vault-drop.test.ts cases)
• npm run build
• npm run release:check
• npm run audit:prod
• Deployed to the local test vault for manual drag-and-drop verification

Safety

• No credentials, private vault content, internal URLs, or personal paths are included
• User-visible changes are documented in CHANGELOG.md