feat: a newer app is told in a snackbar on Home and installed from a sheet holding Settings' card, fetched ahead on Wi-Fi (#6452)

Mirrors Librarian (#6446, #6448), after Gallery's update tray (canon #6367).

- Home shows "Version X of the app is on your server." / "…is downloaded
  and ready." with Update / Install; the button opens a bottom sheet
  holding the same About card Settings shows, so there is one install
  path. Each build is told at most once a day, kept on disk.
- The shell-wide update banner and its in-memory dismissals are gone.
- AppUpdates (singleton) holds the offer and the install, so Home and
  Settings show the same state. Checks run when either screen opens, on
  every return to the front, every 5 minutes while shown, and on Home's
  pull to refresh.
- UpdateFetchWorker downloads the newer APK ahead on an unmetered network
  (battery and storage not low, three tries); the download writes a .part
  file and keeps it under the build's name only once its size and sha256
  match, so Install is usually one tap.
- Back from allowing installs, the update the listener asked for carries
  on (kept in SavedStateHandle) instead of asking them to tap again.
- An offer needs a higher code AND a different name: the same app built
  again at a later minute is not offered.
- Server: /api/client/version reports the APK's sha256, worked out from
  the served file and cached by size and mtime.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/api/client_assets.go

@@ -17,8 +17,11 @@ package api
 // client treats 404 as "no update channel available."
 
 import (
+	"crypto/sha256"
+	"encoding/hex"
 	"encoding/json"
 	"errors"
+	"io"
 	"net/http"
 	"os"
 	"path/filepath"
@@ -62,6 +65,41 @@ var (
 	clientAPKLastDownloadMu sync.Mutex
 )
 
+// clientAPKSum is the bundled APK's sha256, kept until the file changes
+// under it (another size or modification time), so a version check every
+// few minutes from every phone does not re-read tens of megabytes each time.
+type clientAPKSum struct {
+	size int64
+	mod  time.Time
+	hex  string
+}
+
+var (
+	clientAPKSumCache   clientAPKSum
+	clientAPKSumCacheMu sync.Mutex
+)
+
+// clientAPKDigest returns the sha256 of the APK at path, in hex.
+func clientAPKDigest(path string, st os.FileInfo) (string, error) {
+	clientAPKSumCacheMu.Lock()
+	defer clientAPKSumCacheMu.Unlock()
+	c := clientAPKSumCache
+	if c.hex != "" && c.size == st.Size() && c.mod.Equal(st.ModTime()) {
+		return c.hex, nil
+	}
+	f, err := os.Open(path)
+	if err != nil {
+		return "", err
+	}
+	defer func() { _ = f.Close() }()
+	h := sha256.New()
+	if _, err := io.Copy(h, f); err != nil {
+		return "", err
+	}
+	clientAPKSumCache = clientAPKSum{size: st.Size(), mod: st.ModTime(), hex: hex.EncodeToString(h.Sum(nil))}
+	return clientAPKSumCache.hex, nil
+}
+
 func testResetClientAPKRateLimit() {
 	clientAPKLastDownloadMu.Lock()
 	defer clientAPKLastDownloadMu.Unlock()
@@ -115,6 +153,11 @@ type clientVersionResponse struct {
 	Channel   string `json:"channel,omitempty"`
 	APKURL    string `json:"apk_url"`
 	SizeBytes int64  `json:"size_bytes"`
+	// SHA256 is worked out from the file being served, not copied from the
+	// sidecar, so it cannot disagree with the bytes the phone receives. The
+	// app downloads ahead on Wi-Fi and installs later; this is how it knows
+	// the file it kept is the whole build that was offered.
+	SHA256 string `json:"sha256"`
 }
 
 // handleClientVersion returns the bundled Android client version + a
@@ -159,12 +202,19 @@ func (h *handlers) handleClientVersion(w http.ResponseWriter, _ *http.Request) {
 		return
 	}
 
+	sum, err := clientAPKDigest(apkPath, stat)
+	if err != nil {
+		writeErrWithLog(w, h.logger, "client_version: digest apk", err)
+		return
+	}
+
 	writeJSON(w, http.StatusOK, clientVersionResponse{
 		Version:   strings.TrimSpace(sidecar.Name),
 		Code:      sidecar.Code,
 		Channel:   strings.TrimSpace(sidecar.Channel),
 		APKURL:    "/api/client/apk",
 		SizeBytes: stat.Size(),
+		SHA256:    sum,
 	})
 }