feat(studio): real install progress in the store dialog and the wizard

Both surfaces showed an indeterminate spinner for the whole install,
because the hub only offered a unary call. They now follow
HubAdmin/InstallModuleStream:

- The store's install dialog shows the phase in plain language, the
  overall percentage, megabytes while the size is known, and a seconds
  counter. The counter is the second, independent signal: it keeps
  running even if the hub goes quiet, which is what tells 'slow' apart
  from 'stuck'.
- The setup wizard shows the same bar per module instead of a spinner
  inside the button.
- New strings in both locales for the phases, the byte line and the
  elapsed counter.

progress_bar_visual_test writes the four waiting states to
build/progress/ in either theme, so the result can be judged without
launching the desktop app. Writing it turned up a real trap worth
recording: awaiting toImage() directly in a widget test leaves the
binding waiting forever - the file passed in six seconds, then sat
there until the ten-minute timeout failed the whole suite.
tester.runAsync() is the fix. Full suite: 186 passed.

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

lib/l10n/app_en.arb

@@ -507,6 +507,29 @@
     }
   },
   "storeInstallProgressBody": "Fetching, verifying, unpacking…",
+  "installPhaseResolve": "Looking up the module",
+  "installPhaseDownload": "Downloading",
+  "installPhaseInstall": "Verifying and unpacking",
+  "installPhaseDone": "Done",
+  "installBytes": "{done} of {total} MB",
+  "@installBytes": {
+    "placeholders": {
+      "done": {
+        "type": "String"
+      },
+      "total": {
+        "type": "String"
+      }
+    }
+  },
+  "installElapsed": "{seconds} s elapsed",
+  "@installElapsed": {
+    "placeholders": {
+      "seconds": {
+        "type": "int"
+      }
+    }
+  },
   "storeLoadDocs": "Load documentation",
   "storeDocsHint": "README rendered inline; uses the hub's registry token when needed.",
   "storeDocsFetching": "Fetching README…",