Rename yt-offline → Catacomb (crate, binary, data paths, UI)

Full product rename with a one-time data migration so existing installs keep
their library + bundled toolchain:

- Crate/binary: `yt-offline` → `catacomb` (Cargo.toml, deb/rpm assets, PKGBUILD,
  package.sh, the desktop file → catacomb.desktop, launch.json, release CI,
  tests/api.rs CARGO_BIN_EXE_catacomb). Codeberg repo URLs left as-is (real
  addresses); PKGBUILD pins the checkout dir via `$pkgname::`.
- Data paths: DB `yt-offline.db` → `catacomb.db`, venv `~/.local/share/yt-offline`
  → `~/.local/share/catacomb`. `migrate_legacy_paths()` in main.rs adopts the old
  names on first run (renames DB + WAL/SHM sidecars + the venv dir; best-effort,
  no-op once migrated). Crash log + restore-temp + IPC socket names follow suit.
- Display: window/app title, tray, web page titles, feed titles, login + SPA
  wordmarks → "Catacomb". Env override `YT_OFFLINE_RENDERER` → `CATACOMB_RENDERER`.

Verified: builds clean as target/release/catacomb, 128 unit + 11 integration
tests pass, and a scratch-dir run confirms the DB (+wal) and venv are migrated
with bytes preserved.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

catacomb.desktop

@@ -0,0 +1,11 @@
+[Desktop Entry]
+Type=Application
+Name=YT Offline
+GenericName=Video Archiver
+Comment=Self-hosted archive for YouTube, TikTok, Twitch, and more
+Exec=yt-offline
+Icon=yt-offline
+Categories=AudioVideo;Video;Network;
+Keywords=youtube;yt-dlp;archive;backup;download;
+StartupNotify=true
+Terminal=false