Keep CCMA usable when the member store is mounted read-only

The encrypted volume holding the member data can be mounted without write
access, but starting against such a store failed: the housekeeper takes a lock
file before doing anything, so its startup pass died with a PermissionError and
took the whole start with it.

The store is now probed with an actual write once at startup -- permissions,
mount options and filesystem state all matter, and only an attempt covers them
together -- and a read-only store opens as a read-only session. The housekeeper
is skipped rather than attempted, every write inside the repository goes through
one guard that reports ReadOnlyStoreError (a RepositoryError, so the dialogs
already handle it) instead of letting an OS error surface, and the services that
archive into the member file check before they start sending or rendering.

The session says so permanently: a warning banner above the tabs, "NUR LESEN" in
the window title and status bar, and refused actions explaining why. Program
settings still save -- they live in the user's config directory -- while the
store-backed ones are skipped with a notice. "Erneut prüfen" picks up a volume
that was remounted writable without restarting.

A store that was never initialized still fails, but says that creating one needs
write access.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/ccma/ui/splash.py

@@ -121,7 +121,14 @@ class SplashScreen(tk.Toplevel):
                 self.repository.list_members()
                 member_count = len(self.repository.list_member_ids())
                 findings = []
-                if self.run_housekeeper:
+                # A read-only store makes the housekeeper impossible, not just
+                # unwanted: every one of its passes writes. Skipping it keeps the
+                # rest of the startup -- and the whole read-only session -- working.
+                if self.repository.read_only:
+                    self._messages.put(
+                        ("status", "Store ist schreibgeschützt – Hausmeister übersprungen …")
+                    )
+                elif self.run_housekeeper:
                     self._messages.put(("status", "Starte Hausmeister …"))
                     findings = Housekeeper(self.repository, self.housekeeper_settings).run(
                         member_delay=_member_delay_for_splash(