docs: rename yt-offline → Catacomb across README, ROADMAP, guides, packaging

Prose + commands updated to the new name: brand reads "Catacomb", while
binary/path/package references are the lowercase `catacomb` (commands, deb/rpm
names, ~/.local/share/catacomb, catacomb.db, catacomb.desktop). Codeberg repo
URLs left pointing at the existing repo. Also folds in the pending doc edits
from earlier in the session.

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

PKGBUILD

@@ -1,5 +1,5 @@
 # Maintainer: InannaBeloved <anassaeneroi@pm.me>
-pkgname=yt-offline
+pkgname=catacomb
 pkgver=0.1.0
 pkgrel=1
 pkgdesc="Self-hosted archive for YouTube, TikTok, Twitch, Vimeo, Bandcamp, SoundCloud, Odysee and more. Desktop GUI + web UI."
@@ -23,7 +23,10 @@ makedepends=('rust' 'cargo')
 # previously broke the rusqlite bundled-sqlite link; thin LTO in Cargo
 # does the cross-crate inlining we actually want.
 options=('!lto')
-source=("git+https://codeberg.org/anassaeneroi/yt-offline.git#branch=main")
+# Force the checkout dir to $pkgname (catacomb); the Codeberg repo is still
+# named yt-offline, so without the `$pkgname::` prefix the clone dir wouldn't
+# match the `cd "$pkgname"` below.
+source=("$pkgname::git+https://codeberg.org/anassaeneroi/yt-offline.git#branch=main")
 sha256sums=('SKIP')
 
 pkgver() {
@@ -44,9 +47,9 @@ build() {
 package() {
     cd "$pkgname"
 
-    install -Dm755 target/release/yt-offline "$pkgdir/usr/bin/yt-offline"
-    install -Dm644 youtube-backup.desktop "$pkgdir/usr/share/applications/yt-offline.desktop"
-    install -Dm644 icon.png "$pkgdir/usr/share/pixmaps/yt-offline.png"
+    install -Dm755 target/release/catacomb "$pkgdir/usr/bin/catacomb"
+    install -Dm644 catacomb.desktop "$pkgdir/usr/share/applications/catacomb.desktop"
+    install -Dm644 icon.png "$pkgdir/usr/share/pixmaps/catacomb.png"
     install -Dm644 LICENSE "$pkgdir/usr/share/licenses/$pkgname/LICENSE"
     install -Dm644 README.md "$pkgdir/usr/share/doc/$pkgname/README.md"
 }