v0.1.1 — sticky membership

Groups are now sticky: join once, stay until you explicitly leave.

• See anything offered, whenever you come back. Both sides now sync on every reconnect (anti-entropy on neighbor-up), so two machines that are never online at the same time still converge — no re-sharing links, no re-publishing.
• Joining the same link twice is one membership, and groups show their real name ("Holiday photos", not "received files").
• Explicit leave everywhere: iroh-drop leave <id> on the CLI, "Leave" buttons in both apps.
• "New in your groups" in the app: offers you did not fetch (e.g. a card that timed out while you were away) stay listed with a Get button until you fetch them or leave.
• Durability fixes: creating/joining/publishing persists immediately, and the daemon shuts down cleanly on SIGTERM — killing it no longer loses the last seconds of state.

103 tests passing. macOS app rebuilt; Windows build is compile-verified but still has not run on a real Windows machine (same caveat as v0.1.0). First-launch notes from v0.1.0 still apply (right-click, Open on macOS; SmartScreen on Windows).