feat(toolbar): Insert Current Location button (addresses #1559)

[Bennethxyz]

Adds a toolbar button to insert your current location into the compose box.\n\nWhat\n- New visible_toolbar_buttons.location (default true)\n- Button uses the Geolocation API and inserts a geo:lat,lon URI (six decimals)\n- Works in 1:1 and MUC; text already renders via existing geo: support\n\nWhy\n- Addresses #1559 (bounty): make it easy to send current location.\n\nNotes\n- Gracefully handles browsers without geolocation (toast warning)\n- No auto-send; it inserts into draft so users can edit before sending.