fix: session persistence + h3 readability + outline error messages

Session issues fixed:
- Removed auto-draft-only gate for showing unassigned sessions on new posts
  (now shows on any post that has no linked sessions)
- Auto-link unassigned session to current post when user opens it
- Added beforeunload/pagehide flush to persist messages before page close
  (prevents data loss from 700ms debounce not firing)
- Added warning log when session loads with 0 messages for debugging

UI fix:
- Override WP editor h3 shrinkage (11px/uppercase → 15px/normal/white)
- Fix h2/h4-h6 headings in response content for dark theme readability

Outline error messages:
- Separate empty response from parse failure with distinct actionable messages
- Show model name + token count on empty response for debugging
- Reassure user that parse failures are usually one-time issues

assets/css/sidebar.css

@@ -4554,6 +4554,32 @@ input.wpaw-plan-section-check:checked::before {
 /* ===========================
    AUDIT FIXES: Mode Indicator Badge
    =========================== */
+
+/* Override WordPress editor-sidebar h3 shrinkage inside our panel */
+#wp-agentic-writer\:wp-agentic-writer .interface-complementary-area h3,
+#wp-agentic-writer\:wp-agentic-writer h3,
+.wpaw-response-content h3,
+.wpaw-messages-inner h3 {
+    font-size: 15px !important;
+    text-transform: none !important;
+    font-weight: 700 !important;
+    color: #e0e4ea !important;
+    margin-bottom: 0.5em !important;
+    letter-spacing: normal !important;
+}
+
+.wpaw-response-content h2 {
+    font-size: 17px !important;
+    color: #e8ecf2 !important;
+}
+
+.wpaw-response-content h4,
+.wpaw-response-content h5,
+.wpaw-response-content h6 {
+    font-size: 13px !important;
+    color: #d0d5dd !important;
+}
+
 .wpaw-mode-badge {
     display: inline-flex;
     align-items: center;