feat(history): nodb backlog (#114)

Implements all open items from the nodb backlog:

statusbar
- PREF_STATUSBAR_SHOW_DBBACKEND (default ON) gates the [sqlite] /
  [flatfile] indicator; toggle via "/statusbar show|hide dbbackend"

/history off|on
- "/history off" now stops persistence as well as hiding history on
  open: sets PREF_DBLOG=off + PREF_CHLOG=false in addition to
  PREF_HISTORY=false
- "/history on" restores persistence (re-enabling PREF_DBLOG=on if
  it was off) and PREF_CHLOG, in addition to PREF_HISTORY=true

/logging
- Removed the "/logging chat on|off" subcommand — chat-log
  persistence is now controlled exclusively by /history; the
  command kept only "/logging group on|off" for MUC logs
- All "use '/logging chat on' to enable" hints replaced with
  "/history on"

/privacy logging
- Single-pass validation across {on, off, redact, flatfile}
- Backend-switching values (on, flatfile) now take effect
  immediately when connected (via log_database_switch_backend),
  matching "/history switch" behaviour; off/redact only flip
  pref bits and keep the live backend open
- Updated help text to reflect runtime switching

/correction off
- win_print_outgoing and win_print_outgoing_with_receipt now gate
  _win_correct on PREF_CORRECTION_ALLOW, matching incoming-msg
  paths. Previously a peer's correction reflected via XEP-0280
  carbons (or the user's own /correct invocation) was applied
  in-buffer regardless of the pref

style
- Two local g_strndup allocations in database_flatfile.c switched
  from char* with manual g_free to auto_gchar gchar*

src/command/cmd_ac.c

@@ -1119,6 +1119,7 @@ cmd_ac_init(void)
     autocomplete_add(statusbar_show_ac, "name");
     autocomplete_add(statusbar_show_ac, "number");
     autocomplete_add(statusbar_show_ac, "read");
+    autocomplete_add(statusbar_show_ac, "dbbackend");
 
     autocomplete_add(statusbar_tabmode_ac, "actlist");
     autocomplete_add(statusbar_tabmode_ac, "dynamic");
@@ -1133,7 +1134,6 @@ cmd_ac_init(void)
     autocomplete_add(status_state_ac, "xa");
     autocomplete_add(status_state_ac, "dnd");
 
-    autocomplete_add(logging_ac, "chat");
     autocomplete_add(logging_ac, "group");
 
     autocomplete_add(privacy_ac, "logging");
@@ -4044,11 +4044,6 @@ _logging_autocomplete(ProfWin* window, const char* const input, gboolean previou
         return result;
     }
 
-    result = autocomplete_param_with_func(input, "/logging chat", prefs_autocomplete_boolean_choice, previous, NULL);
-    if (result) {
-        return result;
-    }
-
     result = autocomplete_param_with_ac(input, "/logging group", logging_group_ac, TRUE, previous);
     return result;
 }