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>

docs/src/SUMMARY.md

@@ -2,7 +2,7 @@
 
 [Introduction](./introduction.md)
 
-# Using yt-offline
+# Using catacomb
 
 - [Installation](./installation.md)
 - [First run & configuration](./first-run.md)