ADFA-3857: fix Flutter Template plugin imagery (variant thumbnails + plugin icon) (#47)

* ADFA-3857: distinct per-variant thumbnails for Flutter templates

All five Flutter template variants shipped the same placeholder thumb.png
(a generic teal Material-app preview), so the New Project screen showed no
indicator of the KIND of Flutter project. Replace each with a distinct
512x512 thumbnail: a Flutter-blue header (matching the existing card style)
over the state-management logo — Flutter logo (Basic), bloc hexagon (BLoC),
Dart bird (Provider), GetX mark (GetX), Riverpod peaks (Riverpod).

Verified on device RFCT704HEAL: reinstalled the rebuilt .cgp, the plugin
regenerated its .cgt templates, and the New Project grid now renders five
distinct kind indicators.

* ADFA-3857: replace PCF-derived plugin icons with Flutter icons

The plugin's day/night icons (assets/icon_day.png, icon_night.png) were
byte-identical copies of the pebble-custom-function-template-installer icons
used as a model, so the Plugin Manager showed a pink 'PCF' badge for the
Flutter Template plugin. Replace both with a Flutter-branded rounded-square
icon (Flutter logo on the day off-white / night deep-blue-black background).

Verified on device RFCT704HEAL: cleared the Glide icon cache + extracted
app_plugin_icons, reinstalled, and the Plugin Manager now renders the Flutter
logo (night variant) instead of the PCF badge.

* ADFA-3857: add grey perimeter border to Flutter template thumbnails

In light mode the thumbnails' white card bodies bled into the near-white New
Project background, so only the blue header was visible. Add a neutral grey
(#9AA0A6) perimeter stroke around each card so the edge reads against both the
light and dark backgrounds, matching the framing of the built-in templates.

Verified in light mode on device RFCT704HEAL.

* ADFA-3857: bump Flutter Template max_ide_version to 26.51

The declared IDE-compatibility range topped out at 26.28, excluding the
current stable release. Raise plugin.max_ide_version so the plugin advertises
support for current and upcoming CoGo releases.

Author: hal-eisen-adfa

flutter-template/src/main/AndroidManifest.xml

@@ -31,7 +31,7 @@
 
         <meta-data
             android:name="plugin.max_ide_version"
-            android:value="26.28" />
+            android:value="26.51" />
 
         <!--
             Available permissions (comma-separated):