Merge branch 'master' into merge/upstream-full

Bring in master commits since the previous merge:
- 4776c1f1e fix(ai): properly decode \uXXXX JSON escape sequences as UTF-8
- 53cdf488b fix(ai): resolve UAF and require provider argument instead of defaults
- 3b673150b chore(chatlog): disable background message file logging (stage 1)

Conflict in src/chatlog.c: master stubs out all chat_log_* function
bodies (project policy — disable plain-text per-day chatlogs), while
the upstream-merged version in this branch still had upstream's full
implementation. Taking master's version entirely keeps the no-op
chatlog API surface that the ~20 call sites in event/, otr/ and
profanity.c depend on.

src/command/cmd_funcs.c

@@ -10823,21 +10823,14 @@ cmd_ai_start(ProfWin* window, const char* const command, gchar** args)
     if (g_strv_length(args) >= 2) {
         provider_name = args[1];
     }
-    if (g_strv_length(args) >= 3) {
-        model = args[2];
+
+    if (!provider_name) {
+        cons_show_error("Please specify provider name.");
+        return TRUE;
     }
 
-    // Resolve defaults
-    auto_gchar gchar* owned_provider_name = NULL;
-    if (!provider_name) {
-        // Use default provider from preferences
-        auto_gchar gchar* default_provider = prefs_get_string(PREF_AI_PROVIDER);
-        if (default_provider && strlen(default_provider) > 0) {
-            provider_name = default_provider;
-        }
-    }
-    if (!provider_name) {
-        provider_name = "openai"; // Fallback
+    if (g_strv_length(args) >= 3) {
+        model = args[2];
     }
 
     AIProvider* provider = ai_get_provider(provider_name);