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/l10n/app_localizations_de.dart

@@ -1025,11 +1025,40 @@ class AppLocalizationsDe extends AppLocalizations {
 
   @override
   String get flowsRunDialogBody =>
-      'Eingaben als key=value, eine pro Zeile. Klartext-Werte landen als Text. Wert mit `@` voranstellen liest die Datei als Bytes — wie `fai run --input document=@/pfad/zu/datei.pdf`.';
+      'Pro deklariertem Input ein Feld. Bytes-Inputs öffnen einen Datei-Picker; alle anderen sind Klartext. Der Starten-Button aktiviert sich, wenn alle Pflichtfelder gefüllt sind.';
 
   @override
-  String get flowsInputsHint =>
-      'name=Welt\ndocument=@/Users/ich/Dokumente/beispiel.pdf\nstil=drei Stichpunkte';
+  String get flowsLoadingDefinition => 'Inputs werden geladen…';
+
+  @override
+  String get flowsDefinitionFailed =>
+      'Flow-Definition konnte nicht geladen werden';
+
+  @override
+  String get flowsNoInputs =>
+      'Dieser Flow deklariert keine Inputs. Starten klicken, um loszulegen.';
+
+  @override
+  String get flowsTypeBytesPick => 'Datei wählen…';
+
+  @override
+  String get flowsTypeBytesChange => 'Datei ändern';
+
+  @override
+  String get flowsTypeBytesNoFile => 'Keine Datei ausgewählt';
+
+  @override
+  String flowsTypeBytesSize(int n) {
+    return '$n Bytes';
+  }
+
+  @override
+  String get flowsRunErrorTitle => 'Flow-Lauf fehlgeschlagen';
+
+  @override
+  String flowsFileReadFailed(String path, String error) {
+    return '„$path\" konnte nicht gelesen werden: $error';
+  }
 
   @override
   String get flowsRunButton => 'Starten';