fix(backup): make a restored device match the one that was backed up

Found by wiping a working device and reinstalling from scratch. Every
problem here is invisible until you actually do that, which is why a
green test suite and eleven hours of uptime had not surfaced any of them.

**Four enabled plugins vanished on restore.** Weather, stocks, music and
leaderboard have a registry `id` that differs from the `id` in their own
manifest: the registry calls them `weather`, everything else calls them
`ledmatrix-weather`. Installation already prefers the manifest id for the
directory name and warns when the two disagree, so on disk, in
config.json and in a backup they are `ledmatrix-weather` -- but nothing
resolved that in reverse. Restore asked the store for `ledmatrix-weather`
and got "Plugin not found in registry", four times, and the device came
back missing four plugins the user had enabled.

Registry lookup now falls back to matching `plugin_path`, which already
records `plugins/ledmatrix-weather`. Renaming the published ids would
have orphaned `plugin_state.json` entries keyed on the old ones. Exact id
still wins, so a path that collides with another entry's id cannot
shadow it. Against the live registry and a real 28-plugin install this
takes unresolvable directories from five to one -- the one being
starlark-apps, which is genuinely not in the registry.

**Secrets could not be restored at all.** A fresh install left
config_secrets.json group-readable but not group-writable, and the web
interface -- which is what performs a restore -- does not necessarily run
as the owner. Every other file in the backup restored; secrets failed
with EACCES. Now group-writable, so the account running the web UI can
put them back.

**A partial restore reported "Restore had errors" and nothing else.**
That is the same message whether the whole thing failed or it quietly
dropped your API keys. It now names what was restored, what failed, and
which plugins were not reinstalled.

**ytm_auth.json was never in the backup.** It sits in config/ beside the
three files that are, and is pure device-local auth: losing it silently
signs the user out of YouTube Music. Backed up and restored with the
wifi config, which it resembles.

**Backups were written inside the directory a reinstall deletes.**
config/backups/exports is destroyed by the reinstall the user was told to
make it before. Exports now go beside the install, falling back to the
old path when that is not writable.

**The installer reboots without asking in non-interactive mode**, which
the README did not mention -- easy to hit when piping the install, and
alarming when a device you are installing onto disappears. Documented,
with --no-reboot-prompt. Its log also claimed root:ledmatrix while
printing a hardcoded group name rather than the one it used.

Tests: registry resolution gets its own suite, including the collision
case and third-party entries with an empty plugin_path. The existing
round-trip test passed throughout this because its fixture plugin has a
directory name equal to its id -- the one shape that cannot fail -- so it
now carries ytm_auth too.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Udr6MfaFLUPhX5Fgo67Jf5

first_time_install.sh

@@ -1497,13 +1497,20 @@ if [ -f "$PROJECT_ROOT_DIR/config/config_secrets.json" ]; then
     if [ "$SERVICE_USER" = "root" ]; then
         # Service runs as root - set ownership to root so it can read as owner
         chown "root:$LEDMATRIX_GROUP" "$PROJECT_ROOT_DIR/config/config_secrets.json" || true
-        echo "✓ Secrets file permissions set (root:ledmatrix for root service)"
+        echo "✓ Secrets file permissions set (root:$LEDMATRIX_GROUP for root service)"
     else
         # Service runs as regular user - use ACTUAL_USER and rely on group membership
         chown "$ACTUAL_USER:$LEDMATRIX_GROUP" "$PROJECT_ROOT_DIR/config/config_secrets.json" || true
-        echo "✓ Secrets file permissions set ($ACTUAL_USER:ledmatrix)"
+        echo "✓ Secrets file permissions set ($ACTUAL_USER:$LEDMATRIX_GROUP)"
     fi
-    chmod 640 "$PROJECT_ROOT_DIR/config/config_secrets.json"
+    # Group-writable, not just group-readable. The web interface performs
+    # backup restores, and it does not necessarily run as the file's owner: on
+    # a fresh install the secrets file ended up owned by root while the web
+    # service ran as the login user, so restoring a backup failed with
+    # "Permission denied: config_secrets.json" while every other file in the
+    # same backup restored fine. Read-only for the group makes secrets the one
+    # thing a restore cannot put back.
+    chmod 660 "$PROJECT_ROOT_DIR/config/config_secrets.json"
 fi
 
 # Set proper permissions for YTM auth file (readable by all users including root service)