Phase 4a sub-commit 4: AppImageUpdate sidecar + appcast + checkForUpdates()

Wires in the option-(a) sidecar approach: the AppImage carries a
bundled AppImageUpdate AppImage and an embedded update-info string
in the .upd_info ELF section. BackendConnection drives both the
check and the apply via QProcess.

BackendConnection:
  - Q_INVOKABLE checkForUpdates()
        Bundled mode only. Spawns AppImageUpdate.AppImage with
        --check-for-update <APPIMAGE>. Exits 0 → noUpdatesAvailable,
        1 → updatesAvailable, anything else → updateCheckFailed.
        Dev mode: emits updateCheckFailed("…dev-mode only").
  - Q_INVOKABLE applyUpdate()
        Bundled mode only. Spawns AppImageUpdate.AppImage with
        --remove-old <APPIMAGE>. Replaces the running AppImage in
        place; user must restart. Emits updateApplied or
        updateApplyFailed.
  - Sidecar path resolves to applicationDirPath()/AppImageUpdate.AppImage
    by default, overridable via BRIDGE_APPIMAGEUPDATE_BIN.
  - APPIMAGE env (set by the AppImage runtime) determines the target
    file. Outside an AppImage both methods fail loudly.

build-appimage.sh:
  - Auto-downloads AppImageUpdate-x86_64.AppImage into the cached
    tools dir and copies it into AppDir/usr/bin/AppImageUpdate.AppImage.
  - New --update-info flag, forwarded to appimagetool's -u so the
    .upd_info ELF section carries an "zsync|<URL>" string the sidecar
    will fetch.

examples/todo Makefile forwards APPIMAGE_UPDATE_INFO env to the
script as --update-info.

release.yml:
  - Builds the AppImage with APPIMAGE_UPDATE_INFO set to the canonical
    Gitea Releases asset URL for this tag.
  - Installs zsync, runs zsyncmake to generate Todo-x86_64.AppImage.zsync.
  - Generates a JSON appcast (latest.json) with version / url / sha256 /
    size / zsync URL / released_at — useful as an HTTP-fetchable
    fallback for clients that prefer a structured manifest.
  - SHA256SUMS now covers AppImage + zsync + latest.json.
  - Uploads all four assets to the Gitea Release.

AppImage size grows from ~104 MB to ~152 MB with the sidecar bundled.
Embedding verified: objdump shows .upd_info populated with the
expected zsync URL after a local build.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packaging/linux/build-appimage.sh

@@ -36,6 +36,7 @@ CADDYFILE=""
 DESKTOP=""
 ICON=""
 OUTPUT=""
+UPDATE_INFO=""
 
 usage() {
     cat <<USAGE >&2
@@ -62,6 +63,7 @@ while [ $# -gt 0 ]; do
         --desktop)      DESKTOP="$2"; shift 2 ;;
         --icon)         ICON="$2"; shift 2 ;;
         --output)       OUTPUT="$2"; shift 2 ;;
+        --update-info)  UPDATE_INFO="$2"; shift 2 ;;
         -h|--help)      usage ;;
         *)              echo "unknown arg: $1" >&2; usage ;;
     esac
@@ -116,6 +118,13 @@ if [ ! -x "$APPIMAGETOOL" ]; then
         https://github.com/AppImage/appimagetool/releases/download/continuous/appimagetool-x86_64.AppImage
     chmod +x "$APPIMAGETOOL"
 fi
+APPIMAGEUPDATE="$TOOLS_DIR/AppImageUpdate-x86_64.AppImage"
+if [ ! -x "$APPIMAGEUPDATE" ]; then
+    step "Download AppImageUpdate sidecar"
+    curl -fsSL -o "$APPIMAGEUPDATE" \
+        https://github.com/AppImageCommunity/AppImageUpdate/releases/download/continuous/AppImageUpdate-x86_64.AppImage
+    chmod +x "$APPIMAGEUPDATE"
+fi
 # Pre-cache the runtime file appimagetool needs; downloading it on every
 # build was wedging some networks during squashfs.
 RUNTIME_FILE="$TOOLS_DIR/runtime-x86_64"
@@ -135,9 +144,10 @@ trap 'rm -rf "$(dirname "$APPDIR")"' EXIT INT TERM
 
 step "Stage AppDir at $APPDIR"
 mkdir -p "$APPDIR/usr/bin" "$APPDIR/usr/share/$APP_NAME"
-install -m 0755 "$HOST_BIN"   "$APPDIR/usr/bin/$APP_NAME"
-install -m 0755 "$FRANKENPHP" "$APPDIR/usr/bin/frankenphp"
-install -m 0644 "$CADDYFILE"  "$APPDIR/usr/share/$APP_NAME/Caddyfile"
+install -m 0755 "$HOST_BIN"      "$APPDIR/usr/bin/$APP_NAME"
+install -m 0755 "$FRANKENPHP"    "$APPDIR/usr/bin/frankenphp"
+install -m 0755 "$APPIMAGEUPDATE" "$APPDIR/usr/bin/AppImageUpdate.AppImage"
+install -m 0644 "$CADDYFILE"     "$APPDIR/usr/share/$APP_NAME/Caddyfile"
 
 # Symfony tree: rsync without dev artefacts. Ensure composer ran with
 # --no-dev before building; this script doesn't re-run composer install.
@@ -174,7 +184,14 @@ LD_LIBRARY_PATH="" \
     --plugin qt
 
 step "appimagetool — squash AppDir into single-file AppImage"
-"$APPIMAGETOOL" --runtime-file "$RUNTIME_FILE" "$APPDIR" "$OUTPUT"
+APPIMAGETOOL_ARGS=(--runtime-file "$RUNTIME_FILE")
+if [ -n "$UPDATE_INFO" ]; then
+    # Embed AppImageUpdate update-info string (e.g.
+    # "zsync|https://example.com/Todo-x86_64.AppImage.zsync") into the
+    # .upd_info ELF section so the bundled sidecar can find new releases.
+    APPIMAGETOOL_ARGS+=(-u "$UPDATE_INFO")
+fi
+"$APPIMAGETOOL" "${APPIMAGETOOL_ARGS[@]}" "$APPDIR" "$OUTPUT"
 
 echo
 echo "✓ AppImage at $OUTPUT"