fix: make the update checker visible -- persistent banner + a menu item

Reported from the field: running v1.2 against a repo with v1.3.0 published
gave no prompt, and there appeared to be no way to check manually. The
checker itself works; it was invisible, for two reasons.

#78 -- the notice was written to the shared status label, which 21 other
call sites rewrite. The check runs off-thread and lands a second or two
after launch, right as the user starts clicking, so the next selection or
refresh wiped it. Exactly the bug fixed for the MSIX warning in #35, which
got a persistent banner; that fix was never carried to the update notice.

Adds NoticeBanner: a persistent, dismissible notice carrying its own action
button. It's a shared widget rather than a second bespoke banner, so the
next thing needing the user's attention doesn't reach for the status bar
again. (The MSIX banner still uses its own QLabel -- migrating it is a
follow-up, deliberately not bundled with a bug fix.)

#79 -- the only 'Check for updates' affordance was a button inside the
About dialog, which is not where anyone looks. Worse, the About action was
created without a menu role, and Qt auto-assigns AboutRole to actions whose
text begins with 'About', relocating it into the macOS application menu --
so the notice's own hint, 'Help > About to view it', pointed at a menu that
on macOS doesn't contain the item.

Help now has its own 'Check for updates...' item with an explicit
ApplicationSpecificRole, and the About action states its AboutRole rather
than inheriting it invisibly. The menu-driven check is never throttled and
always reports back -- the user asked, so silence would read as broken.

The decision and the wording live in core.update_notice() because the test
suite has no PySide6 (CI installs pytest + cryptography only), so anything
in bcc.py is untestable. A test asserts the notice text names no menu path,
which is what went stale here in the first place.

Closes #78
Closes #79

bcc_core.py

@@ -163,6 +163,39 @@ def fetch_latest_release(timeout: float = 4.0) -> dict | None:
     return {"version": tag, "url": payload.get("html_url") or RELEASES_URL}
 
 
+def update_notice(
+    current: str, release: dict | None, url_fallback: str = RELEASES_URL
+) -> dict | None:
+    """Decide whether to tell the user about a release, and what to say.
+
+    Returns {"version", "text", "url"} when `release` is newer than `current`,
+    or None when it isn't, when the check failed, or when the payload is
+    malformed. Kept here rather than in the GUI so the wording and the
+    should-we-notify decision are testable -- bcc.py can't be imported by the
+    test suite, which has no PySide6.
+
+    The text deliberately names no menu path. The old status-line notice read
+    "Help > About to view it", which is wrong on macOS: Qt relocates the About
+    action into the application menu (#79). A notice that carries its own
+    action can't drift out of sync with the platform.
+    """
+    if not isinstance(release, dict):
+        return None
+    version = release.get("version")
+    if not version or not isinstance(version, str):
+        return None
+    if not is_newer_version(current, version):
+        return None
+    # Tags carry a "v" prefix and __version__ doesn't; render both the same way
+    # so the notice doesn't read "Version v1.3.0 ... you're running 1.2.0".
+    shown = version.lstrip("vV")
+    return {
+        "version": version,
+        "text": f"Version {shown} is available. You're running {current.lstrip('vV')}.",
+        "url": release.get("url") or url_fallback,
+    }
+
+
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