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/data/flow_output.dart

@@ -0,0 +1,85 @@
+// FlowOutput — typed representation of one entry in
+// `SubmitResponse.outputs`. Lets Studio render text vs. JSON vs.
+// bytes vs. file with the right widget instead of stringifying
+// everything into `<bytes: …>` placeholders.
+
+import 'dart:convert';
+import 'dart:typed_data';
+
+import 'package:fai_dart_sdk/fai_dart_sdk.dart' show Payload;
+
+/// One named output of a flow run. Sealed via factory constructors
+/// — callers pattern-match with `switch` on the runtime type to
+/// pick a renderer. Variants:
+///
+///   - [FlowOutputText]    : plain text. `mimeType` may be set
+///                           (e.g. `text/markdown`) so the
+///                           renderer can pick a Markdown widget.
+///   - [FlowOutputJson]    : a proto Struct, already pretty-
+///                           printed as JSON so the UI can render
+///                           it directly.
+///   - [FlowOutputBytes]   : raw bytes + MIME. The renderer
+///                           decides whether to inline a preview
+///                           (e.g. images) and exposes a
+///                           Save-As affordance otherwise.
+///   - [FlowOutputFile]    : a URI the host already wrote to. The
+///                           renderer offers an Open action.
+///   - [FlowOutputUnknown] : oneof was empty / unset. Never
+///                           expected in practice but kept so
+///                           the UI never panics on a future
+///                           schema bump.
+sealed class FlowOutput {
+  const FlowOutput();
+
+  factory FlowOutput.fromPayload(Payload p) {
+    if (p.hasText()) {
+      return FlowOutputText(text: p.text);
+    }
+    if (p.hasJson()) {
+      // proto Struct → toProto3Json gives a Dart-side
+      // Map/List/primitive tree. We re-encode with an indent so
+      // the UI can show the operator-friendly form directly.
+      final tree = p.json.toProto3Json();
+      const encoder = JsonEncoder.withIndent('  ');
+      return FlowOutputJson(pretty: encoder.convert(tree));
+    }
+    if (p.hasBytes()) {
+      return FlowOutputBytes(
+        bytes: Uint8List.fromList(p.bytes.data),
+        mimeType: p.bytes.mimeType,
+      );
+    }
+    if (p.hasFile()) {
+      return FlowOutputFile(uri: p.file.uri, mimeType: p.file.mimeType);
+    }
+    return const FlowOutputUnknown();
+  }
+}
+
+class FlowOutputText extends FlowOutput {
+  final String text;
+  const FlowOutputText({required this.text});
+}
+
+class FlowOutputJson extends FlowOutput {
+  /// Already indented JSON, ready for SelectableText / Markdown
+  /// code-block rendering.
+  final String pretty;
+  const FlowOutputJson({required this.pretty});
+}
+
+class FlowOutputBytes extends FlowOutput {
+  final Uint8List bytes;
+  final String mimeType;
+  const FlowOutputBytes({required this.bytes, required this.mimeType});
+}
+
+class FlowOutputFile extends FlowOutput {
+  final String uri;
+  final String mimeType;
+  const FlowOutputFile({required this.uri, required this.mimeType});
+}
+
+class FlowOutputUnknown extends FlowOutput {
+  const FlowOutputUnknown();
+}