feat: Add Password-Protected Channel Auto-Discovery UX

[kwsantiago]

Add Password-Protected Channel Auto-Discovery

Description

Closes #177

Adds automatic discovery of password-protected channels when encrypted messages are received. Channels now appear in a "Discovered Channels" section with lock icons (🔒) and clicking them shows a password prompt.

Changes

• Auto-detect password-protected channels from encrypted messages
• Add "Discovered Channels" section to sidebar with lock icons
• Fix password prompt dialog to properly show when clicking discovered channels
• Add visual distinction between joined and discovered channels

Screenshots

• Before: Users had to manually type /join #secret password
• After: Click discovered channel → password prompt → join

Testing

1. On another bitchat device, create a new channel and add a password to it: /pass mypassword
2. On that other bitchat device, send message in that newly created password-protected channel
3. Check Android - channel appears in "Discovered Channels" with 🔒
4. Click channel - password prompt appears
5. Enter password - join and see decrypted messages

Privacy

• No broadcasting since it only discovers from actual messages
• All discovery is local to device
• No protocol changes

Code Quality

• Ran ./gradlew lint - existing baseline warnings only
• No new tests added - UI feature tested manually with bitchat android

Checklist

• I have read the contribution guidelines: https://github.com/permissionlesstech/bitchat-android?tab=readme-ov-file#contributing
• I have performed a self-review of my code

• I have mentioned the corresponding issue and the relevant keyword (e.g., "Closes: #xy") in the description (see https://docs.github.com/en/issues/tracking-your-work-with-issues/linking-a-pull-request-to-an-issue)
• If it is a core feature, I have added automated tests