Migrate legacy conversation notification settings to channels

[RankoR]

• Migrate legacy per-conversation notification preferences into Android conversation notification channels once at app startup.
• Preserve existing per-conversation mute, ringtone, and vibration settings during migration without overwriting already-created channels.
• Stop runtime conversation-channel creation from reading legacy database notification fields, so new channels inherit the parent incoming-message channel defaults.
• Open conversation notification settings through the parent incoming-message channel plus EXTRA_CONVERSATION_ID.
• Add focused Robolectric coverage for the migration, channel creation behavior, and conversation-settings intent handling.

Fixes #97