Add Android share-sheet routine sharing

[SamDozer]

Once the export format exists, wire it to Android's share sheet so a routine can
go out through any app the user already has.

• Share a routine from the routine list and from the editor
• Receive a shared routine file through an intent filter
• Preview before accepting — the recipient sees what they are about to add
• Import from a file picker too, for anyone who saved it first

The preview matters. Importing something blindly into your own practice library
is not acceptable.

Depends on #3. Part of v0.2.