feat(studio): Cmd+K palette, recent-activity panel, icon/screenshot rendering (v0.21.0)

Three sweeps in a single bundle.

Universal command palette (⌘K / ctrl+K):
- Modal at top-of-window with a single search input. Indexes
  pages, installed modules, store entries, saved flows on
  open; filters client-side as the operator types.
- Keyboard nav (↑/↓ + Enter), grouped sections, hover-to-
  highlight. Hit selection navigates / opens the right
  surface in one keystroke. Closes with Esc.
- Designed to mirror the VSCode / Linear / 1Password
  ergonomics — gives non-CLI operators a "jump anywhere"
  affordance that scales with the number of installed
  modules.

Modules page recent-activity panel:
- Top-of-page strip lists the last 10 install / uninstall
  events from the audit log, color-coded by direction.
  Hidden when no relevant events exist (fresh installs).
  Same locale-unambiguous timestamp format used in audit.

Store detail sheet now renders icons + screenshots:
- `_ModuleIcon` widget loads the explicit `iconUrl` if
  provided, falls back to the category glyph on missing
  URL or load failure (no broken-image rectangle).
- Screenshots strip below the description: 320×200 tiles,
  horizontally-scrollable, click opens full-size via the
  OS handler. Placeholder card on load failure.

StoreItem extended with `iconUrl`, `screenshotUrls`,
`docsUrl` so the new content paths through unchanged.

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: 0.20.1
+version: 0.21.0
 
 environment:
   sdk: ^3.11.0-200.1.beta