fix(worktrees): scope base branches to multi-root repositories

[im-ian]

Summary

This fixes multi-root project worktree creation so each repository root uses its own configured base branch.

1. Repository-aware settings — Load and preserve project settings for every root while keeping general and pull-request settings attached to the current repository.
2. Explicit root selection — Add a searchable repository-root selector to the Worktrees tab and load branch choices for the selected root.
3. Safe per-root persistence — Save only changed source-root settings, preserving unrelated settings fields and the primary root base branch.
4. Regression coverage — Verify distinct branch lists and base-branch updates across primary and source repositories.

Expected impact

Area	Before	After
Multi-root worktree creation	Source roots could inherit the primary repository base branch	Each repository root uses its own base branch
Project settings UI	The affected repository was implicit	The target repository root is explicit and searchable
Existing settings	Updating another root risked conflating settings	General and pull-request settings remain preserved per root

Design notes

• Base branches remain stored in the existing per-repository settings records because valid refs can differ between roots.
• The selector displays the full repository path so roots with identical directory names remain distinguishable.
• Branch requests are cancelled logically when the selected root changes, preventing stale responses from replacing the current list.

Test plan

• pnpm run typecheck — passed
• pnpm run test — 1,290 tests passed
• pnpm exec playwright test tests/e2e/project-settings.spec.ts — 10 tests passed
• pnpm run test:e2e — 318 tests passed
• pnpm run build — passed
• git diff --check — passed

Notes

• The existing Vite chunk-size warning remains and is addressed separately from this behavior fix.

[jtf-ian]

Merge readiness note: this PR is technically mergeable now.

• GitHub reports MERGEABLE / CLEAN.
• CI is green: Frontend, E2E (shard 1/2), and E2E (shard 2/2) passed.
• Self-review is complete with no blocking findings.
• fix label is applied.

Proceeding to merge because the user explicitly requested the reviewed release fixes to be merged.