feat: LAN update check on startup with notify banner (stage 2)

Read manifest.json from the configured update_source and show a dismissable
banner when a newer version is available. Notify-only — no install yet.

- services/update_service.py: version compare + check_for_update (pure, tested)
- config: add update_source key (empty = no check)
- main_window: top banner, background-thread check; open folder via os.startfile
  (QDesktopServices.openUrl mishandles file:// folder URLs — ShellExecute err 2)
- tests: +15 covering version compare and check_for_update branches
- docs 02/05/10 + tasks 17.16

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/services/config_service.py

@@ -11,6 +11,7 @@ DEFAULT_CONFIG = {
     "last_print_dir": "",
     "last_template": "",       # name of the last-selected template
     "last_batch_mode": "full_combo",  # BatchMode value of the last-used mode
+    "update_source": "",       # LAN folder holding manifest.json (empty = no update check)
 }
 
 _CONFIG_FILENAME = "app_config.json"

src/services/update_service.py

@@ -0,0 +1,93 @@
+"""LAN update check (stage 2: notify-only).
+
+Reads a manifest.json from a configured LAN folder and reports whether a newer
+version is advertised. This module performs no installation — it only decides
+whether to notify the user. See docs/10-lan-update.md.
+
+All functions degrade gracefully: a missing/unreachable source or a malformed
+manifest yields "no update" rather than an error, so an update check can never
+block or break startup.
+"""
+import json
+import logging
+from dataclasses import dataclass
+from pathlib import Path
+from typing import Optional
+
+logger = logging.getLogger(__name__)
+
+MANIFEST_NAME = "manifest.json"
+
+
+@dataclass
+class UpdateInfo:
+    """A newer version advertised by the update source."""
+    version: str
+    source: str            # folder the user opens to get the new version
+    notes: str = ""
+    mandatory: bool = False
+
+
+def parse_version(text) -> tuple:
+    """Parse 'a.b.c' into a comparable (a, b, c) int tuple.
+
+    Tolerant: missing parts pad with 0, non-numeric suffixes (e.g. '1rc2')
+    keep their leading digits, fully non-numeric parts become 0.
+    """
+    nums = []
+    for part in str(text).strip().split(".")[:3]:
+        digits = ""
+        for ch in part:
+            if ch.isdigit():
+                digits += ch
+            else:
+                break
+        nums.append(int(digits) if digits else 0)
+    while len(nums) < 3:
+        nums.append(0)
+    return tuple(nums)
+
+
+def is_newer(remote, local) -> bool:
+    """True if version string *remote* is strictly newer than *local*."""
+    return parse_version(remote) > parse_version(local)
+
+
+def check_for_update(update_source, current_version) -> Optional[UpdateInfo]:
+    """Return UpdateInfo if *update_source* advertises a version newer than
+    *current_version*, else None.
+
+    Returns None (never raises) when:
+    - no source is configured,
+    - the source/manifest is unreachable or unreadable,
+    - the manifest is malformed,
+    - the advertised version is not newer.
+    """
+    if not update_source:
+        return None
+
+    manifest_path = Path(update_source) / MANIFEST_NAME
+    try:
+        with open(str(manifest_path), encoding="utf-8") as f:
+            data = json.load(f)
+    except (OSError, ValueError) as exc:
+        logger.info("Update check skipped (%s): %s", type(exc).__name__, exc)
+        return None
+
+    if not isinstance(data, dict):
+        logger.warning("Manifest is not a JSON object, ignoring: %s", manifest_path)
+        return None
+
+    version = str(data.get("version", "")).strip()
+    if not version or not is_newer(version, current_version):
+        return None
+
+    source = str(data.get("source", "")).strip() or str(update_source)
+    info = UpdateInfo(
+        version=version,
+        source=source,
+        notes=str(data.get("notes", "")),
+        mandatory=bool(data.get("mandatory", False)),
+    )
+    logger.info("Update available: v%s (current v%s)", version, current_version)
+    return info