Fix wide-char (emoji/CJK) column drift through the whole render chain

Wide characters shifted everything after them one column to the right:

- render.rs: vt100's empty wide-continuation cell was padded to a
  space, making wide chars occupy 2+1 columns — skip it entirely
- ipc.rs buffer_to_ansi: same bug for daemon frames — skip cells
  covered by a preceding wide symbol (unicode-width)
- app.rs: clamp virtual terminals to >= 2 columns (vt100 0.16 has a
  subtraction underflow panic on wide chars in a 1-col grid) and give
  content_area a sane 80x24 default so windows spawned before the
  first layout pass aren't degenerate
- main.rs: use try_send for display frames so one slow/stalled client
  can no longer freeze the whole WM
- integration test: new end-to-end check that emoji do not shift the
  row, plus fix a false match against the window title

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/integration.py

@@ -172,6 +172,22 @@ def main():
         text = disp.frame_text(timeout=5.0, contains="x")
         check("key_event når den virtuella terminalen", "x" in text)
 
+        # ── 4b. Breda tecken (emoji) får inte förskjuta raden ───────────
+        # vt100 lämnar en tom fortsättningscell efter breda tecken; om
+        # renderaren gör den till mellanslag blir "📁X" → "📁 X" och allt
+        # efter emojin skjuts åt höger.
+        # OBS: fönstertiteln visar kommandosträngen (innehåller "X-MARKER"
+        # som text) — vänta därför på emojin+markören som bara finns i
+        # fönstrets INNEHÅLL.
+        app.send({"type": "spawn_window",
+                  "command": "printf '\\360\\237\\223\\201X-MARKER\\n'; exec cat",
+                  "request_id": "w-emoji"})
+        emoji_open = app.recv_until("window_opened")
+        text = disp.frame_text(timeout=5.0, contains="📁X-MARKER")
+        check("emoji förskjuter inte raden", "📁X-MARKER" in text,
+              f"frame-text: {text[-200:]!r}")
+        app.send({"type": "close_window", "window_id": emoji_open["id"]})
+
         # ── 5. Popup + popup_result via Enter ───────────────────────────
         app.send({
             "type": "spawn_popup", "message": "Integrationstest?",