feat: handle SEND intents for contact imports

[naveensingh]

Type of change(s)

• Bug fix
• Feature / enhancement
• Infrastructure / tooling (CI, build, deps, tests)
• Documentation

What changed and why

Added handling for android.intent.action.SEND in MainActivity. It relies on the same import logic as when open VCF files from other apps. For simplicity, this only supports single VCF shares and doesn't handle android.intent.action.SEND_MULTIPLE (proposed in #383).

The importer could be a different/alias activity with the Import contacts label, but this is also nice, and I'm lazy.

Tests performed

• Importing contacts by sharing and opening from other apps.

Closes the following issue(s)

• Closes sharing a Contact doesn't offer Fossify Contacts #321

Checklist

• I read the contribution guidelines.
• I manually tested my changes on device/emulator (if applicable).
• I updated the "Unreleased" section in CHANGELOG.md (if applicable).
• I have self-reviewed my pull request (no typos, formatting errors, etc.).
• All checks are passing.