v0.1.2 — UX polish

A UX polish round on both apps, and the version numbers now match the release tags.

macOS

• Consent notifications carry Accept/Decline actions — answer with the window closed; banners withdraw themselves when answered or expired.
• New "File received" notification with Show in Downloads.
• Try Again on failed transfers, context menus on every row (Copy Link, Show Link & QR…, Show in Finder, Leave/Stop).
• The menu-bar icon reflects what's happening, and the menu now shows "New in your groups" with Get buttons.
• Consent cards name the group; progress shows "1.2 of 3.4 MB"; dismissable errors; ⌘O to send; the app is called Drop everywhere.

Windows / Linux (egui)

• Framed Send/Receive sections, progress text, Try Again, "Copied ✓" feedback, drag-hover tint, dismissable errors, an empty state, per-group Link buttons, and Get buttons that can't be double-tapped.
• Bug fix: joining an empty group blocked the UI up to 60s and then errored. An empty group is just a group whose files haven't been offered yet — the join now returns quickly and offers arrive when they arrive.

103 tests passing, clippy clean. Same caveat as before: the Windows build is compile-verified but has not been run on real Windows hardware yet (this release is what we'll test with). First-launch notes: right-click → Open on macOS; SmartScreen → More info → Run anyway on Windows.