[Stretch] Add export conversation history (JSON download)

**Why:** Users may want to backup or export their transcription history.

**What to build:** Add "Export" button to conversation view. Downloads JSON file with all transcriptions, replies, timestamps, and profiles. Format: { version: "1.0", conversations: [...] }. Privacy notice (local download, no upload).

**Acceptance criteria:**
- [ ] "Export Conversations" button in UI
- [ ] Downloads JSON file (chat-copilot-export-YYYY-MM-DD.json)
- [ ] JSON includes all conversations from IndexedDB
- [ ] Schema: { version, exportedAt, conversations: [{ id, transcriptions, replies, profile, timestamp }] }
- [ ] Privacy notice: "Downloaded locally, never uploaded"
- [ ] Import feature out of scope (stretch++)

**Depends on:** #32

Start at the conversation view and inspect how conversations and profiles are stored in IndexedDB. Trace the data needed for transcriptions, replies, timestamps, and profiles, then add the export flow with the specified filename, schema, and privacy notice. Done means the UI downloads all stored conversations as JSON without uploading them.

Written by the indexing model from the issue text.