FRE-4680: Implement Milestone 2 advanced features

- Draft auto-save with configurable interval, status, and manual trigger
- Email template system (save, list, use, delete templates)
- Export messages to JSON, MBOX, or EML format
- Import messages from JSON files
- Register new commands (thread, bulk, export, import, autosave, template)
- Update README.md with documentation for all new commands

Build and tests pass clean.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

README.md

@@ -12,6 +12,10 @@ A ProtonMail CLI tool written in Go, similar to gog.
 - **Webhook Management**: Real-time notifications via webhook subscriptions
 - **External PGP Key Management**: Import, export, encrypt, decrypt, sign, and verify with external PGP keys
 - **CLI Plugin System**: Extend Pop functionality with external plugins
+- **Email Threading**: View and reply to conversation threads
+- **Bulk Operations**: Delete, trash, star, and mark read/unread multiple messages at once
+- **Export/Import**: Export messages to JSON, MBOX, or EML format; import from JSON
+- **Draft Auto-Save**: Automatic draft saving with configurable interval and email templates
 
 ## Installation
 
@@ -119,6 +123,85 @@ pop plugin enable myplugin
 pop plugin disable myplugin
 ```
 
+### Email Threading
+
+```bash
+# List conversation threads
+pop thread list
+
+# View a full conversation thread
+pop thread show <conversation-id>
+
+# Reply to a conversation
+pop thread reply <conversation-id> --body "Your reply here"
+pop thread reply <conversation-id> --body-file reply.txt
+```
+
+### Bulk Operations
+
+```bash
+# Delete multiple messages
+pop bulk delete --ids "id1,id2,id3"
+pop bulk delete --ids-file ids.txt
+
+# Move multiple messages to trash
+pop bulk trash --ids "id1,id2,id3"
+
+# Star multiple messages
+pop bulk star --ids "id1,id2,id3"
+pop bulk unstar --ids "id1,id2,id3"
+
+# Mark messages as read/unread
+pop bulk mark-read --ids "id1,id2,id3"
+pop bulk mark-unread --ids "id1,id2,id3"
+```
+
+### Export and Import
+
+```bash
+# Export specific messages
+pop export messages --ids "id1,id2,id3" --output messages.json
+pop export messages --ids-file ids.txt --output messages.json
+
+# Export messages by search
+pop export messages --search "important" --output search-results.json
+
+# Export all messages from a folder
+pop export folder --folder inbox --output inbox-backup.json
+pop export folder --folder sent --format mbox --output sent.mbox
+
+# Export a conversation
+pop export conversation <conversation-id> --output conversation.json
+
+# Import messages from JSON
+pop import --file messages.json
+```
+
+### Draft Auto-Save
+
+```bash
+# Enable auto-save with default 30-second interval
+pop draft autosave enable
+
+# Enable with custom interval
+pop draft autosave enable --interval 60
+
+# Disable auto-save
+pop draft autosave disable
+
+# Check auto-save status
+pop draft autosave status
+
+# Manually trigger auto-save
+pop draft autosave run
+
+# Manage email templates
+pop draft template save newsletter --subject "Monthly Update" --body "Hello..."
+pop draft template list
+pop draft template use newsletter --to recipient@example.com
+pop draft template delete newsletter
+```
+
 ## Project Structure
 
 ```
@@ -135,8 +218,11 @@ pop/
 │   ├── accounts.go   # Multi-account support
 │   ├── webhook.go    # Webhook management
 │   ├── pgp.go        # PGP key management
-│   ├── plugin.go     # Plugin management
-│   └── thread.go     # Thread management
+│   ├── plugin.go       # Plugin management
+│   ├── thread.go       # Conversation threading
+│   ├── bulk.go         # Bulk operations
+│   ├── export.go       # Export/import functionality
+│   └── draft_autosave.go # Draft auto-save and templates
 ├── internal/
 │   ├── auth/         # Session management
 │   │   └── session.go