fix(terminal): bail the backpressure refresh on a mid-fetch tab switch

The refresh can now issue two fetches (full history, then the tail as a
downgrade fallback), which widens an existing window where the user switches
tabs mid-flight and this session's history gets painted into the terminal they
are now looking at. Guard it the way _maybeRefetchFullHistory already does.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/web/public/app.js

@@ -2213,6 +2213,10 @@ class CodemanApp {
         res = await fetch(`/api/sessions/${sessionId}/terminal?tail=${TERMINAL_TAIL_SIZE}`);
         data = (await res.json())?.data ?? {};
       }
+      // Bail on a tab switch mid-fetch: writing here would paint this session's
+      // history into the terminal the user is now looking at. The window is two
+      // fetches wide in the fallback case, so this guard is not optional.
+      if (this.activeSessionId !== sessionId) return;
       if (data.terminalBuffer) {
         // This refresh is SERVER-triggered, so a user quietly reading scrollback
         // did not ask for it and must not be dragged to the bottom by it (#259).
@@ -2225,7 +2229,7 @@ class CodemanApp {
         await this.chunkedTerminalWrite(data.terminalBuffer);
         // A tail fetch can be partial, and the banner would otherwise keep
         // describing the pre-refresh buffer (#258).
-        if (this.activeSessionId === sessionId) this._setHistoryTruncation(sessionId, data);
+        this._setHistoryTruncation(sessionId, data);
         const target = computeRewriteScrollLine({
           linesFromBottom,
           baseY: this.terminal.buffer?.active?.baseY ?? 0,