feat(studio): typed run-flow form, file picker, copyable errors (v0.41.0)

The free-form key=value run-flow dialog produced two
unhelpful failures every time an operator hit Run:

  1. Submit empty → "step references missing value
     '\$inputs.document'" from the hub. Studio sent a zero-
     entry inputs map because nothing told the operator the
     flow declared a required input.
  2. Type `document=@/path/to/file` → "path not found".
     Dart's `File()` doesn't expand `~`, and on macOS
     sandboxed Studio can't read arbitrary paths anyway.

Both failure modes are gone. The dialog is now a typed form:

- On open, fetches `getFlowDefinition(name)` (new SDK
  v0.15.0 wrapper around the v0.10.89 hub RPC). While that
  resolves, a small spinner shows
  "Loading inputs…"; on failure, an inline `FaiErrorBox`
  with the exact RPC error and a copy button replaces the
  spinner.
- Renders one form-field per declared input. The flow
  YAML's verbatim type tag drives the widget choice:
  `bytes` / `file` → "Choose file…" button + picked-file
  readout, plain TextField for everything else. The type
  tag is shown next to the input name as a pill so a flow
  author who picks a less-common type still gets a hint.
- Bytes inputs route through `file_picker` with
  `withData: true`, which means the OS file dialog handles
  read access — sandboxed Studio gets the bytes inline
  rather than a path it can't open. Falls back to
  `File(path).readAsBytes()` on Linux configs that don't
  honour `withData` for large files; failures show a
  copyable SnackBar.
- The Run button enables only when every declared input has
  a value (text non-empty / file picked). Empty submission
  is impossible; the cryptic hub-side
  "missing value" error stops surfacing.
- The run-result dialog's error path uses `FaiErrorBox`
  instead of the previous plain `SelectableText`, so any
  hub-side flow error (permission denied, module crash,
  whatever) is one click to clipboard.

11 new ARB keys cover the form's labels, the file-picker
states, the loading / failed-definition messages, the
"This flow declares no inputs" empty state, and the
run-error dialog title.

`file_picker: ^8.0.0` added to pubspec.yaml.

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

lib/data/hub.dart

@@ -546,6 +546,15 @@ class HubService {
       ..sort((a, b) => a.name.compareTo(b.name));
   }
 
+  /// Return the parsed input schema of a saved flow. Used by
+  /// Studio's Flows page to render a typed run-flow form
+  /// before calling [runSavedFlow]. Each entry's `type` is
+  /// the verbatim type tag from the flow YAML (`text`,
+  /// `bytes`, `json`, …).
+  Future<List<FlowInputDef>> getFlowDefinition(String name) async {
+    return _client.getFlowDefinition(name);
+  }
+
   /// Run a saved flow with the supplied inputs. Text values
   /// land as text Payloads, byte values as bytes Payloads —
   /// flows that mix both (e.g. extract taking a `document: