feat: toggle file tree expand/collapse with state-aware button

Related Issue

N/A

Changes

• File tree header button now toggles between Expand All (when all collapsed) and Collapse All (when any folder is open), with state-driven icon and aria-label.
• Replaced the previous custom chevron icon with the VSCode collapse-all codicon for visual consistency.
• Lifted folder open/close state from individual TreeNodeItem up to FileTree (openPaths: Set<string>) so the header button can drive bulk expand/collapse.
• Added expandAll translation keys to locales/ko.json and locales/en.json; threaded expandAllLabel through Sidebar → FileTree.
• Added key and explicit fallback={null} to the <Suspense> wrapping BranchSelector to satisfy React's child-key validation when the element is forwarded as a prop across the server/client boundary.

How to Verify

1. npm run dev and open any repository page (/[lang]/repository/[owner]/[repo]).
2. Confirm the sidebar header shows a + icon when the tree is fully collapsed.
3. Click it — every directory expands and the icon switches to −.
4. Click again — every directory collapses and the icon returns to +.
5. Toggle a single folder manually and verify the header icon stays as − as long as at least one folder is open.
6. Hover the button in both states and confirm the tooltip / aria-label shows the localized "Expand All" / "Collapse All" (KO and EN).
7. Open DevTools console and verify there is no Each child in a list should have a unique "key" prop warning.

Checklist

• npm run format:check passes
• npm run lint passes
• npm run typecheck passes
• npm run build passes
• PR title follows Conventional Commits (feat:, fix:, chore:, refactor:, docs:, test:, )