feat(studio): localised + explainer-rich Add module source dialog

The previous Add-source dialog was English-only and
operator-hostile — answered 'where do I install from?' without
explaining what a private module *is*. New version, DE + EN:

  - Title + intro + button labels routed through AppLocalizations
    ("Modul-Quelle hinzufügen" / "Installation fehlgeschlagen"
    instead of raw English strings).
  - 'How private modules work' explainer block (3 sentences):
    what a module is (`module.yaml` + WASM), how to package
    it (`fai pack <dir>` → .fai bundle), where to host it
    (any URL: own Forgejo / GitHub / S3), what verification the
    hub does (sha256 + signature against trust store).
  - Honest about the Studio gap: the dialog can install URLs +
    packed bundles, but unpacked source directories still
    require `fai install --link <path>` on the CLI. The
    example command lives in its own code-style box, selectable.
  - Layout: SingleChildScrollView'd so the explainer doesn't
    push the buttons off short viewports.

Studio bumped to 0.66.0; editor path-override pulls in 0.20.0.

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

lib/l10n/app_en.arb

@@ -1451,5 +1451,25 @@
       "count": {"type": "int"}
     }
   },
-  "doctorPathStudioErrors": "Studio errors"
+  "doctorPathStudioErrors": "Studio errors",
+  "addSourceTitle": "Add module source",
+  "addSourceIntro": "`{capability}` is not in the public store. Point the hub at a `.fai` bundle URL or a local bundle path; the hub downloads, verifies (sha256 + signature) and installs it.",
+  "@addSourceIntro": {
+    "placeholders": {
+      "capability": {"type": "String"}
+    }
+  },
+  "addSourceField": "URL or path to .fai bundle",
+  "addSourceHint": "https://git.flemming.ai/your-org/your-module/releases/download/v0.1.0/foo-0.1.0.fai",
+  "addSourceHowItWorksTitle": "How private modules work",
+  "addSourceHowItWorksBody": "A module is a directory with a module.yaml + the WASM artifact. To share it: pack it (`fai pack <dir>`) and host the resulting `.fai` bundle anywhere (your own Forgejo / GitHub / S3). The hub installs by URL and verifies the signature against its trust store.\n\nDeveloping locally? Use the CLI — Studio cannot install from an unpacked directory (yet):",
+  "addSourceCliExample": "fai install --link /path/to/module",
+  "addSourceInstallButton": "Install",
+  "addSourceCancel": "Cancel",
+  "installFailed": "Install failed: {error}",
+  "@installFailed": {
+    "placeholders": {
+      "error": {"type": "String"}
+    }
+  }
 }