chore(flutter): pin our own git plugins instead of tracking their HEAD

dash_chat_2, window_manager and desktop_multi_window named only a url, so
the lock recorded `ref: HEAD` for them. That holds while the lock is used
verbatim, but any re-resolution -- an unrelated pubspec.yaml edit, a lock
conflict resolved by regenerating -- re-reads HEAD and silently moves the
plugin to whatever the fork's tip is that day. All three forks are ours and
get pushed to, which is how window_manager and desktop_multi_window drifted
across five lock bumps since July with no pubspec.yaml change to show for it.

Each ref is the resolved-ref already in the lock, and all three still match
their fork's HEAD, so nothing resolves differently today.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EZ49AbZJYfm8NTp5yDPMab

flutter/pubspec.lock

@@ -306,7 +306,7 @@ packages:
     dependency: "direct main"
     description:
       path: "."
-      ref: HEAD
+      ref: bd6b5b41254e57c5bcece202ebfb234de63e6487
       resolved-ref: bd6b5b41254e57c5bcece202ebfb234de63e6487
       url: "https://github.com/rustdesk-org/Dash-Chat-2"
     source: git
@@ -339,8 +339,8 @@ packages:
     dependency: "direct main"
     description:
       path: "."
-      ref: HEAD
-      resolved-ref: 8b774a66671cbb9bcb2631af6ac28f9bdd469ce3
+      ref: "8b774a66671cbb9bcb2631af6ac28f9bdd469ce3"
+      resolved-ref: "8b774a66671cbb9bcb2631af6ac28f9bdd469ce3"
       url: "https://github.com/rustdesk-org/rustdesk_desktop_multi_window"
     source: git
     version: "0.1.0"
@@ -1581,7 +1581,7 @@ packages:
     dependency: "direct main"
     description:
       path: "."
-      ref: HEAD
+      ref: cf4aef0512092fad9344a27ffe1c47ad83269dfc
       resolved-ref: cf4aef0512092fad9344a27ffe1c47ad83269dfc
       url: "https://github.com/rustdesk-org/window_manager"
     source: git

flutter/pubspec.yaml

@@ -40,6 +40,7 @@ dependencies:
   dash_chat_2:
     git:
       url: https://github.com/rustdesk-org/Dash-Chat-2
+      ref: bd6b5b41254e57c5bcece202ebfb234de63e6487
   draggable_float_widget: ^0.1.0
   settings_ui: ^2.0.2
   flutter_breadcrumb: ^1.0.1
@@ -53,9 +54,11 @@ dependencies:
   window_manager:
     git:
       url: https://github.com/rustdesk-org/window_manager
+      ref: cf4aef0512092fad9344a27ffe1c47ad83269dfc
   desktop_multi_window:
     git:
       url: https://github.com/rustdesk-org/rustdesk_desktop_multi_window
+      ref: 8b774a66671cbb9bcb2631af6ac28f9bdd469ce3
   freezed_annotation: ^2.0.3
   flutter_custom_cursor:
     git: