Local data foundation: conversation & recording management (before cloud sync)

[YJack0000]

Before cloud sync (planned separately), the local management layer for conversations and their audio should be solid. The local SQLite store already exists; this adds the user-facing management on top of it.

Why do this first:

• It's fully local — benefits every direct-download user, no account needed.
• Cloud sync will sync exactly these records + media, so clean local models and lifecycle make sync straightforward.
• Recording retention/size matters for privacy and disk before anything leaves the device.

Sub-issues

• Conversation library — list, search, rename, tag, delete, export (Local conversation library: list, search, rename, tag, delete, export #8)
• Recording (audio) management — storage, playback, retention, disk usage (Local recording (audio) management: playback, retention, disk usage #9)

Principles

• Local-first: everything here works offline with no account.
• Every entity keeps updated_at + soft-delete so the future sync layer can adopt it without a migration.
• Deleting a conversation must also handle its audio file (and vice versa) — no orphans.