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/format.dart

@@ -0,0 +1,33 @@
+// Display-only formatters shared across pages. Pure functions,
+// no I/O, no locale fall-through — locale-specific labels (the
+// unit word itself, e.g. "Bytes" vs. "bytes") stay in `.arb`;
+// these helpers only handle the numeric formatting.
+
+/// Render a byte count with an SI suffix that stays at three
+/// significant digits ("23.3 kB", "1.04 MB", "847 B"). Uses 1000
+/// as the base, not 1024 — matches macOS Finder / GNOME Files
+/// and is what most operators expect when they read "kB".
+///
+/// Returns plain `n B` for values under 1000 so small flows
+/// and tiny inputs stay precise instead of becoming "0.5 kB".
+String humanBytes(int bytes) {
+  if (bytes < 1000) return '$bytes B';
+  const units = ['kB', 'MB', 'GB', 'TB', 'PB'];
+  var v = bytes.toDouble() / 1000.0;
+  var unitIndex = 0;
+  while (v >= 1000 && unitIndex < units.length - 1) {
+    v /= 1000.0;
+    unitIndex++;
+  }
+  // Pick decimals so the rendered number stays at 3 significant
+  // figures: 9.99 / 99.9 / 999 — same approach Finder uses.
+  final String formatted;
+  if (v >= 100) {
+    formatted = v.toStringAsFixed(0);
+  } else if (v >= 10) {
+    formatted = v.toStringAsFixed(1);
+  } else {
+    formatted = v.toStringAsFixed(2);
+  }
+  return '$formatted ${units[unitIndex]}';
+}