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*

tests/unittests/test_cmd_otr.c

@@ -69,7 +69,7 @@ cmd_otr_log_on_shows_warning_when_chlog_disabled(void** state)
     prefs_set_boolean(PREF_CHLOG, FALSE);
 
     expect_cons_show("OTR messages will be logged as plaintext.");
-    expect_cons_show("Chat logging is currently disabled, use '/logging chat on' to enable.");
+    expect_cons_show("Chat logging is currently disabled, use '/history on' to enable.");
 
     gboolean result = cmd_otr_log(NULL, CMD_OTR, args);
     assert_true(result);
@@ -115,7 +115,7 @@ cmd_otr_log_redact_shows_warning_when_chlog_disabled(void** state)
     prefs_set_boolean(PREF_CHLOG, FALSE);
 
     expect_cons_show("OTR messages will be logged as '[redacted]'.");
-    expect_cons_show("Chat logging is currently disabled, use '/logging chat on' to enable.");
+    expect_cons_show("Chat logging is currently disabled, use '/history on' to enable.");
 
     gboolean result = cmd_otr_log(NULL, CMD_OTR, args);
     assert_true(result);