feat(studio): inline flow editor — YAML highlighting + Run (v0.50.0)

New top-level destination "Editor" (Cmd+5) ships as Studio's
fifth surface. The editor reads + writes flow YAML directly
under ~/.fai/data/flows/ via dart:io — the hub picks up the
changes on the next listFlows / runSavedFlow call.

Layout: two-pane shell. Left (240 px) is the file list; right
flexes to the code pane and an optional results column when
Run produces output. Top toolbar exposes:
  * filename + dirty-mark
  * New flow (scaffolds from a debug.echo template)
  * Save (writes the active file to disk)
  * Run (saves first if dirty, calls
    HubService.runSavedFlow, surfaces typed FlowOutputs in
    a side panel)
  * Refresh

YAML highlighting via flutter_code_editor + the highlight
package's yaml language. Lightweight style map mapping the
five token classes that actually appear in flow YAML
(attr / string / number / comment / subst for the
${{ ... }} template syntax) to FaiTheme colors — keeps the
editor visually consistent with the rest of Studio.

New-flow naming uses a FilteringTextInputFormatter that
restricts the name to [a-z0-9_-]. A "name already exists"
SnackBar surfaces the conflict instead of silently
overwriting.

Bilingual strings shipped (en.arb + de.arb) for every
operator-facing string: toolbar buttons, dialogs, empty
states, file-exists error, run-output header.

New deps:
  * flutter_code_editor ^0.3.5
  * highlight (transitive — pinned as direct so the
    yaml-language import has its declared dependency).

Smoke-test (test/flow_editor_test.dart) pumps the page and
asserts the empty-state + toolbar render without throwing
on hosts that don't have ~/.fai/data/flows yet. The full
file-list + open-on-tap flow needs a writable HOME override
which dart:io's read-only Platform.environment doesn't allow
inside a test isolate — that path lives in the integration
suite as a follow-up.

Version bumps:
  * pubspec.yaml: 0.49.1 → 0.50.0
  * main.dart kStudioVersion: 0.42.0 → 0.50.0 (had drifted
    behind pubspec; brought back into sync as part of this
    bump)

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

lib/l10n/app_en.arb

@@ -1384,5 +1384,37 @@
       }
     }
   },
-  "storeSourceTemporal": "Temporal"
+  "storeSourceTemporal": "Temporal",
+
+  "navFlowEditor": "Editor",
+  "flowEditorNew": "New flow",
+  "flowEditorSave": "Save",
+  "flowEditorRun": "Run",
+  "flowEditorRefresh": "Refresh file list",
+  "flowEditorRunOutput": "RUN OUTPUT",
+  "flowEditorEmptyTitle": "No flow open",
+  "flowEditorEmptyBody": "Pick one from the left, or click New flow to start a fresh one.",
+  "flowEditorListEmptyTitle": "No saved flows",
+  "flowEditorListEmptyBody": "Click New flow to scaffold one from a template.",
+  "flowEditorDiscardTitle": "Discard unsaved changes?",
+  "flowEditorDiscardBody": "Switching files will throw away the edits you haven't saved.",
+  "flowEditorDiscardKeep": "Keep editing",
+  "flowEditorDiscardThrow": "Throw away",
+  "flowEditorNewDialogTitle": "New flow",
+  "flowEditorNewDialogLabel": "Flow name",
+  "flowEditorNewDialogHelper": "lowercase letters, digits, hyphen, underscore",
+  "flowEditorNewDialogCancel": "Cancel",
+  "flowEditorNewDialogCreate": "Create",
+  "flowEditorNewTemplateComment": "Generated by Studio's Flow editor. Edit, save, run.",
+  "@flowEditorNewTemplateComment": {
+    "placeholders": {
+      "name": {"type": "String"}
+    }
+  },
+  "flowEditorAlreadyExists": "A flow named \"{name}\" already exists.",
+  "@flowEditorAlreadyExists": {
+    "placeholders": {
+      "name": {"type": "String"}
+    }
+  }
 }