DAN-67: Make arbitrary branch comparison discoverable

[danielss-dev]

Compare branches was buried in ref context menus, required a local checkout, and seeded the HEAD menu with the first other ref. Add Compare branches… to palette Actions and Branch actions, and share deliberate defaults: current vs upstream, then main/master, then the first available pair. Current stays on the To side; detached HEAD can compare any two available branches without checkout.

Local, remote, and tag context menus remain available while detached. The palette opens after its focus restoration completes. All entry points reuse CompareRefsDialog → tauri.repoDiffBetween → Repo::diff_between; Commits multi-select Compare is preserved. Update the README, user guide, and planning documents.

Linear: DAN-67

Validation:

• pnpm --filter ./ui exec tsc --noEmit — passed.
• pnpm --filter ./ui test — 482 tests passed across 86 files, including 13 focused defaults tests.
• git diff --check — passed.
• Commands ran through npx --yes pnpm@9.0.0; a frozen-lockfile install restored the existing Pierre dependency patch.
• Native UI interaction was not exercised in this Linux environment.

After / Screenshots

Captured from the Strand web demo (vite --mode demo against the in-browser acme-api sample repo) on this PR head. Native Tauri desktop was not available here.

Entry points

CompareRefsDialog