feat(studio): typed flow outputs + on-disk docs + UX polish

Documentation system (the original "whole docs system is broken"
complaint):
- Studio reads inline docs from disk via the new
  getInstalledModuleDocs RPC. No network, no auth, no provider
  lock-in.
- store.dart probes for MODULE.md eagerly when a module-detail
  sheet opens (a single file stat). The Documentation section
  only renders when the bundle actually shipped docs.
- Shared FaiTheme.markdownStyle helper used by Welcome's
  DocReaderSheet and Store's DocsPanel so every inline doc
  reads in the same typography. The shared style forces
  code.backgroundColor = transparent to suppress the per-span
  bands flutter_markdown's default code style draws on dark
  themes.

Flow run dialog:
- Run-button gating now strips the @version suffix from
  installed capabilities before the contains() check. Without
  this fix the button stayed disabled for every flow with
  dependencies, even after a successful install.
- Studio derives a MIME type from the picked file's extension
  (small per-suffix map; .pdf, .docx, .txt, .json, ...) and
  forwards it to the hub. Fixes "unsupported MIME type:
  application/octet-stream" from text.extract.
- Dialog title tracks the future's state: running -> "extract
  laeuft", success -> "extract -- Ergebnis", failure ->
  "extract -- fehlgeschlagen".
- Output rendering moved from String stringification to a
  sealed FlowOutput type (Text / Json / Bytes / File / Unknown).
  A new FaiFlowOutput widget dispatches per variant: markdown
  for text/markdown (heuristic), pretty JSON for proto Struct,
  inline image preview + Save-As for bytes, Open for file URIs.

Byte sizes:
- New humanBytes() helper renders 23.3 kB / 1.04 MB style values
  with three significant digits, matching Finder / GNOME Files.
  Wired into flow-card pills, picked-file readouts, and the
  bytes-payload preview line.

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

lib/pages/welcome.dart

@@ -1058,17 +1058,7 @@ class _DocReaderSheetState extends State<_DocReaderSheet> {
                       await SystemActions.openInOs(href);
                     }
                   },
-                  styleSheet: MarkdownStyleSheet.fromTheme(theme).copyWith(
-                    p: theme.textTheme.bodyMedium?.copyWith(height: 1.5),
-                    code: FaiTheme.mono(
-                      size: 12,
-                      color: theme.colorScheme.onSurface,
-                    ),
-                    codeblockDecoration: BoxDecoration(
-                      color: theme.colorScheme.surfaceContainerHigh,
-                      borderRadius: BorderRadius.circular(FaiRadius.sm),
-                    ),
-                  ),
+                  styleSheet: FaiTheme.markdownStyle(theme),
                 );
               },
             ),