feat(dictation): ⌘D push-to-talk, stop button icon when recording

- ⌘D is now push-to-talk: hold to record, release to stop.
  AppShell dispatches keydown/keyup events; useComposerDictation
  starts on keydown and stops on keyup.
- Button click still toggles (click mic to start, click stop to end).
- Mic icon swaps to a filled stop square while recording.
- Shortcut description updated to reflect hold-to-talk behavior.

desktop/src/app/AppShell.tsx

@@ -597,7 +597,7 @@ export function AppShell() {
 
       if (key === "d" && !event.shiftKey) {
         event.preventDefault();
-        window.dispatchEvent(new CustomEvent("buzz:toggle-dictation"));
+        window.dispatchEvent(new CustomEvent("buzz:dictation-key-down"));
         return;
       }
 
@@ -608,9 +608,17 @@ export function AppShell() {
       }
     }
 
+    function handleKeyUp(event: KeyboardEvent) {
+      if (event.key.toLowerCase() === "d") {
+        window.dispatchEvent(new CustomEvent("buzz:dictation-key-up"));
+      }
+    }
+
     window.addEventListener("keydown", handleKeyDown);
+    window.addEventListener("keyup", handleKeyUp);
     return () => {
       window.removeEventListener("keydown", handleKeyDown);
+      window.removeEventListener("keyup", handleKeyUp);
     };
   }, [
     handleOpenBrowseChannels,

desktop/src/features/dictation/hooks/useComposerDictation.ts

@@ -71,16 +71,31 @@ export function useComposerDictation({
     }
   }, [disabled, dictation.isRecording, dictation.cancelRecording]);
 
-  // ⌘D global shortcut — dispatched from AppShell's keydown handler.
+  // ⌘D push-to-talk — hold to record, release to stop.
+  // Dispatched from AppShell's keydown/keyup handlers.
   useEffect(() => {
-    function handleToggle() {
-      dictation.toggleRecording();
+    function handleKeyDown() {
+      if (!dictation.isRecording && !dictation.isStarting) {
+        dictation.startRecording();
+      }
     }
-    window.addEventListener("buzz:toggle-dictation", handleToggle);
+    function handleKeyUp() {
+      if (dictation.isRecording || dictation.isStarting) {
+        dictation.stopRecording();
+      }
+    }
+    window.addEventListener("buzz:dictation-key-down", handleKeyDown);
+    window.addEventListener("buzz:dictation-key-up", handleKeyUp);
     return () => {
-      window.removeEventListener("buzz:toggle-dictation", handleToggle);
+      window.removeEventListener("buzz:dictation-key-down", handleKeyDown);
+      window.removeEventListener("buzz:dictation-key-up", handleKeyUp);
     };
-  }, [dictation.toggleRecording]);
+  }, [
+    dictation.isRecording,
+    dictation.isStarting,
+    dictation.startRecording,
+    dictation.stopRecording,
+  ]);
 
   return dictation;
 }

desktop/src/features/dictation/ui/DictationButton.tsx

@@ -1,4 +1,4 @@
-import { Mic } from "lucide-react";
+import { Mic, Square } from "lucide-react";
 import { Button } from "@/shared/ui/button";
 import { Tooltip, TooltipContent, TooltipTrigger } from "@/shared/ui/tooltip";
 import { cn } from "@/shared/lib/cn";
@@ -51,7 +51,11 @@ export function DictationButton({
           type="button"
           variant={dictation.isRecording ? "default" : "ghost"}
         >
-          <Mic />
+          {dictation.isRecording ? (
+            <Square className="h-3.5 w-3.5 fill-current" />
+          ) : (
+            <Mic />
+          )}
         </Button>
       </TooltipTrigger>
       <TooltipContent>

desktop/src/shared/lib/keyboard-shortcuts.ts

@@ -184,7 +184,7 @@ export const KEYBOARD_SHORTCUTS: KeyboardShortcut[] = [
   {
     id: "voice-dictation",
     label: "Voice Dictation",
-    description: "Start or stop voice dictation in the composer",
+    description: "Hold to dictate, release to stop (push-to-talk)",
     keys: "⌘D",
     keysWindows: "Ctrl+D",
     category: "Messages",