feat(android): sync device albums with a server album

Add phone album sync to the Android client: MediaStore source (device
albums), a Room index with upload provenance, on-device previews, and a
mixed server + device album list where a linked pair collapses into a
single device card with a link indicator and mode label.

Three modes over one pipeline: A (off), B (upload copies, originals stay
local), C (upload originals and offload them locally to previews). The
sync engine tracks device changes (add/remove/rename/move/content), matches
assets to server photos by sha256 content hash, deletes only this client's
uploads, and reflects server changes without touching local originals.
Offload is automatic; the zoomed viewer pulls the server original.

Backend: expose the original content checksum in photo data (Postgres and
SQLite), additive with no migration.

Also anchor the `.gitignore` `data/` rule to `/data/` so the Android
`com.galo.android.data` package is tracked, and add the halo launcher icon.

.gitignore

@@ -15,7 +15,9 @@ __pycache__/
 # Local data backends (defaults)
 galo.db
 galo.db-*
-data/
+# Root-level local FS storage (FS_ROOT=./data); anchored so it does not match
+# the Android source package android/.../com/galo/android/data/.
+/data/
 
 # Android build output and local toolchain config
 android/.gradle/