feat(studio): inline README + per-module update badge in Store (v0.19.0)

Two Phase-2 Store improvements that operators feel
immediately:

- Per-module update badge: store cards compare the operator's
  installed version against the index's `best_version`, raise
  an "update" pill + "Update to vX.Y.Z" button when newer is
  available. Result: a single glance at the Store reveals
  what's behind. The page now fetches `listModules` in
  parallel with the search so the comparison is on the
  current snapshot.

- Inline README rendering: detail sheet has a new
  Documentation section. "Load documentation" button (lazy —
  no network until clicked) calls FetchModuleDocs and renders
  the markdown via flutter_markdown, with the operator's
  theme colors and link-tap routed through SystemActions.
  Auth / not-found / network failures fall back to a
  friendly fix-hint panel + "Open repository in browser"
  button so the operator never hits a dead end.

Adds flutter_markdown ^0.7.7 to pubspec; the package is
maintenance-only upstream but is the only mature option
for Material-themed markdown right now. Easy to swap to
markdown_widget if/when we need GitHub-flavored extras.

Signed-off-by: flemming-it <stefan.a.flemming@googlemail.com>

pubspec.yaml

@@ -1,7 +1,7 @@
 name: fai_studio
 description: "F∆I Studio — desktop GUI for the F∆I hub"
 publish_to: 'none'
-version: 0.18.0
+version: 0.19.0
 
 environment:
   sdk: ^3.11.0-200.1.beta
@@ -16,6 +16,8 @@ dependencies:
     path: ../fai_dart_sdk
   google_fonts: ^8.1.0
   shared_preferences: ^2.5.5
+  # Inline README rendering inside the Store detail sheet.
+  flutter_markdown: ^0.7.7
 
 dev_dependencies:
   flutter_test: