core: extract the store and sync engine into a shared crate (M12 step 1)

Android becomes a native Kotlin client over this same code (Scribe note 2730), so
the local store and sync engine stop being modules of the desktop app and become
`thoughtsync-core`, a crate with no UI framework in it at all.

This is a move, not a rewrite, and the measurement is why: every file in local/
and sync/ already carried ZERO Tauri references — 4,980 of 6,372 lines. The
coupling was 473 lines of command shim, which stays behind in the desktop crate
as src/commands/. Kept as git renames so history follows the files.

The desktop imports them under their old names (`use thoughtsync_core::{local,
sync}`) so every call site reads exactly as before. What moved is where they
live, not what they are.

Two things a workspace changes that are easy to miss, both caught before pushing:

[profile.release] now lives at the workspace ROOT. Cargo silently ignores
profiles declared by a non-root member — leaving it in the desktop crate would
have dropped lto/strip/opt-level from every release build with only a warning.

And a workspace shares ONE target dir, so the bundles moved from
desktop/src-tauri/target to target/. Thirteen references across publish-release,
debundle-graphics, verify.sh, package-prebuilt and the workflow now point there.
Pinning target-dir back would have been the smaller diff, but the Android lane
also produces Rust artifacts and they do not belong under desktop/.

Also retires the Tauri Android lane in the same push rather than leaving a path
that is being replaced: gen/android, android.yml and docs/android-dev.md are
gone, the mobile_entry_point attribute with them, and the lib drops to rlib —
staticlib/cdylib existed for Tauri mobile, and the .so Android loads will be
built from the core crate instead. Rule 22, no parallel path.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

.forgejo/workflows/desktop.yml

@@ -20,6 +20,13 @@ on:
     tags: ["v*"]
     paths:
       - "desktop/**"
+      # The shared client core (store + sync engine) the desktop wraps. Its own
+      # crate since the Android client binds the same code, so a change there is a
+      # change to this app even though nothing under desktop/ moved.
+      - "core/**"
+      # The workspace manifest and lockfile, which now live at the repo root.
+      - "Cargo.toml"
+      - "Cargo.lock"
       # The whole frontend, not just the adapter/bridge seam: it is compiled INTO
       # the desktop binary, so any part of it changing means the shipped app is out
       # of date. Config and lockfile included — a dependency bump changes the bundle
@@ -142,7 +149,6 @@ jobs:
           cargo tauri signer sign "$appimage"
           [ -s "$appimage.sig" ] || { echo "ERROR: re-signing produced no .sig" >&2; exit 1; }
           echo "Re-signed $(basename "$appimage")"
-        working-directory: desktop/src-tauri
 
       # install.sh hands the .deb to every Debian/Ubuntu user, so the package's
       # Depends must be right BEFORE a release exists. Prints the generated
@@ -180,9 +186,9 @@ jobs:
         with:
           name: thoughtsync-linux
           path: |
-            desktop/src-tauri/target/release/bundle/appimage/*.AppImage
-            desktop/src-tauri/target/release/bundle/deb/*.deb
-            desktop/src-tauri/target/release/bundle/arch/*.pkg.tar.*
+            target/release/bundle/appimage/*.AppImage
+            target/release/bundle/deb/*.deb
+            target/release/bundle/arch/*.pkg.tar.*
           # error, not warn: a build that bundles nothing should report as a
           # failure, not as a green run with an empty artifact.
           if-no-files-found: error
@@ -296,7 +302,7 @@ jobs:
         uses: https://git.fabledsword.com/bvandeusen/upload-artifact@cb8afe72b42edc798abfb8fcb556cf660d894245
         with:
           name: thoughtsync-windows
-          path: desktop/src-tauri/target/x86_64-pc-windows-msvc/release/bundle/nsis/*.exe
+          path: target/x86_64-pc-windows-msvc/release/bundle/nsis/*.exe
           if-no-files-found: error
 
       # Publishes to the SAME release as the Linux job. Safe to run twice: the