Fix missing titles on existing GUI conversations

[yxlyx]

What changed

• Recognize both user-text content forms when saving session titles, skip empty and peer-injected content, and discard stale placeholders on resume.
• Recover legacy GUI titles consistently across listing, search, and opening a conversation. Invalidate cached recovery when the saved file changes.
• Add regression and edge-case coverage for persistence, malformed and image-only messages, Unicode boundaries, cache rewrites, and API consistency.

Why

Existing conversations could be saved as "Untitled session" because the title fallback ignored structured user text. Correcting persistence prevents new occurrences; display-time recovery repairs existing entries without rewriting conversations or replacing named titles. Legacy reads remain bounded, and malformed or oversized files retain their saved fallback.

Verification

Desktop unit tests and production build passed. The complete tier-1 pre-push checks passed.

Fixes #830.

[justrach]

Follow-up that keeps this commit (author credit) and unblocks the red checks: #859.

zig 0.17 rejects ** next to ++ in the UTF-8 wrap fixtures, so every check on this head failed to compile. #859 rewrites those fixtures with memset + slices. It does not force-push this branch.