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/config/preferences.c

@@ -2208,6 +2208,7 @@ _get_group(preference_t pref)
     case PREF_STATUSBAR_SHOW_NAME:
     case PREF_STATUSBAR_SHOW_NUMBER:
     case PREF_STATUSBAR_SHOW_READ:
+    case PREF_STATUSBAR_SHOW_DBBACKEND:
     case PREF_STATUSBAR_SELF:
     case PREF_STATUSBAR_CHAT:
     case PREF_STATUSBAR_ROOM_TITLE:
@@ -2535,6 +2536,8 @@ _get_key(preference_t pref)
         return "statusbar.show.number";
     case PREF_STATUSBAR_SHOW_READ:
         return "statusbar.show.read";
+    case PREF_STATUSBAR_SHOW_DBBACKEND:
+        return "statusbar.show.dbbackend";
     case PREF_STATUSBAR_SELF:
         return "statusbar.self";
     case PREF_STATUSBAR_CHAT:
@@ -2628,6 +2631,7 @@ _get_default_boolean(preference_t pref)
     case PREF_ROOM_LIST_CACHE:
     case PREF_STATUSBAR_SHOW_NUMBER:
     case PREF_STATUSBAR_SHOW_READ:
+    case PREF_STATUSBAR_SHOW_DBBACKEND:
     case PREF_REVEAL_OS:
     case PREF_CORRECTION_ALLOW:
     case PREF_RECEIPTS_SEND: