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_de.arb

@@ -389,8 +389,23 @@
   "flowsNoneHint": "Flow speichern mit `fai admin flows save <name> <flow.yaml>`, dann taucht er hier auf.",
   "flowsRunDialogTitle": "{name} ausführen",
   "@flowsRunDialogTitle": { "placeholders": { "name": { "type": "String" } } },
-  "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`.",
-  "flowsInputsHint": "name=Welt\ndocument=@/Users/ich/Dokumente/beispiel.pdf\nstil=drei Stichpunkte",
+  "flowsRunDialogBody": "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.",
+  "flowsLoadingDefinition": "Inputs werden geladen…",
+  "flowsDefinitionFailed": "Flow-Definition konnte nicht geladen werden",
+  "flowsNoInputs": "Dieser Flow deklariert keine Inputs. Starten klicken, um loszulegen.",
+  "flowsTypeBytesPick": "Datei wählen…",
+  "flowsTypeBytesChange": "Datei ändern",
+  "flowsTypeBytesNoFile": "Keine Datei ausgewählt",
+  "flowsTypeBytesSize": "{n} Bytes",
+  "@flowsTypeBytesSize": { "placeholders": { "n": { "type": "int" } } },
+  "flowsRunErrorTitle": "Flow-Lauf fehlgeschlagen",
+  "flowsFileReadFailed": "„{path}\" konnte nicht gelesen werden: {error}",
+  "@flowsFileReadFailed": {
+    "placeholders": {
+      "path": { "type": "String" },
+      "error": { "type": "String" }
+    }
+  },
   "flowsRunButton": "Starten",
   "flowsRunningTitle": "{name} läuft",
   "@flowsRunningTitle": { "placeholders": { "name": { "type": "String" } } },