refactor(app): remove unused UI helpers

[kitlangton]

What
Remove confirmed-unused private app helper and controller surface. This is a callerless cleanup with no reachable UI behavior change.

How

• Remove layout.review.closePath and layout.review.togglePath, introduced with the session helper cleanup in dce7eceb28; retain the actively used review.openPath and review.setOpen APIs.
• Remove file.tree.toggle, introduced with the file tree in d9eed4c6ca; retain active explicit tree.expand and tree.collapse behavior.
• Remove mobileSidebar.show from the controller introduced in b8872d9d20; retain active mobileSidebar.hide and mobileSidebar.toggle behavior.
• Remove sanitizeProject, added during the global sync refactor in 2c58dd6203; retain normalizeProjectInfo, including current-project canonical worktree and VCS normalization.
• Remove formatKeybindKeys, added with app UI improvements in 1bcb9d7cb9; retain active formatKeybindParts formatting for KeybindV2 consumers.
• Remove getSessionKey, introduced in dce7eceb28; retain SessionStateKey.from(scope, route) construction so session identities remain server-scoped.

Scope

• No changes to the unrelated Canvas/path closePath mock in packages/app/happydom.ts.
• No session or timeline behavior changes, so no behavioral benchmark was run.
• No demo: the removed APIs had no callers and produce no reachable UI change.

Testing

• bun typecheck from packages/app (pass)
• repository pre-push typecheck across 33 packages (pass)
• bun run build from packages/app (pass; existing Vite chunk/dynamic-import warnings)
• bun run test:unit from packages/app (688 pass, 1 unrelated existing failure: Arabic i18n parity is missing five English keys)
• git diff --check (pass)