fix(desktop): only the exact channel route keeps a live switch trace

Blind-review round 13 (Sonnet xhigh) hardening: leavesChannelSurface
used a /channels/ prefix check, so sibling routes (forum posts) counted
as staying on the channel surface and kept a live trace alive. No
currently reachable path turns that into a wrong record — every
re-entry overwrites or drops the trace — but the invariant from
9b462328d should hold structurally, not incidentally. Anything other
than the exact message-view route now drops the active trace.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Signed-off-by: Max Lampert <maxwell@squareup.com>

desktop/src/app/navigation/useAppNavigation.ts

@@ -54,8 +54,10 @@ export function useAppNavigation() {
         hasStateUpdate: next.state !== undefined,
         // Leaving the channel surface must drop any active switch trace —
         // including one whose channel screen never mounted (route still
-        // resolving), which no component cleanup can cover.
-        leavesChannelSurface: !next.to.startsWith("/channels/"),
+        // resolving), which no component cleanup can cover. Only the exact
+        // channel message-view route keeps a live trace: sibling routes
+        // (forum posts) mount different, untraced screens.
+        leavesChannelSurface: next.to !== "/channels/$channelId",
         navigate: () =>
           navigate({
             ...next,