fix: OTR/presence/OMEMO correctness, stanza-id disco gate, build hardening

- OTR: strip the whitespace tag by shifting the full message tail incl. the NUL, not tag_length bytes, so the body is no longer duplicated for messages longer than the tag.
- presence: snapshot the resource fields before connection_add_available_resource() takes ownership, removing a use-after-free in the own-presence path.
- OMEMO: propagate _omemo_finalize_identity_load() failure on connect (log + cons_show_error + stop) instead of leaving OMEMO silently unavailable.
- OMEMO: guard NULL fingerprint decode in _omemo_fingerprint_decode / omemo_is_trusted_identity / omemo_trust and log every decode failure instead of failing silently.
- stanza-id: gate XEP-0359 dedup on disco urn:xmpp:sid:0 (the `by` JID or its domain), falling back to no-dedup when caps are unknown; add functional tests for trusted vs untrusted server.
- accounts: narrow the group-name sanitizer to the characters GKeyFile forbids in headers ([ ] \n \r), keeping `=` and `#`, so read/write stays symmetric.
- build: re-introduce compiler/sanitizer flags in a Pikaur-safe form (opt-in sanitizers, -Wsign-compare) and fix the resulting -Wsign-compare warnings (incl. proftest _mkdir_recursive).fix: OTR/presence/OMEMO correctness, stanza-id disco gate, build hardening

Author: jabber.developer2 <jabber.developer2@jabber.space>

src/database_flatfile.c

@@ -88,7 +88,7 @@ _ff_cache_line_ids(ff_contact_state_t* state, const char* line)
         return;
 
     // Split metadata on unescaped '|'
-    auto_gchar gchar* meta_str = g_strndup(bracket + 1, close - bracket - 1);
+    auto_gchar gchar* meta_str = g_strndup(bracket + 1, g_diff_to_gsize(close, bracket + 1));
     char** parts = ff_split_meta(meta_str);
 
     char* stanza_id = NULL;
@@ -111,9 +111,9 @@ _ff_cache_line_ids(ff_contact_state_t* state, const char* line)
         const char* sender_start = close + 2;
         const char* colonspace = ff_find_unescaped_colonspace(sender_start);
         if (colonspace) {
-            const char* slash = memchr(sender_start, '/', colonspace - sender_start);
+            const char* slash = memchr(sender_start, '/', g_diff_to_gsize(colonspace, sender_start));
             const char* jid_end = slash ? slash : colonspace;
-            from_jid = g_strndup(sender_start, jid_end - sender_start);
+            from_jid = g_strndup(sender_start, g_diff_to_gsize(jid_end, sender_start));
         }
     }
 
@@ -140,7 +140,7 @@ _ff_maybe_index_line(ff_contact_state_t* state, const char* buf, off_t pos)
     if (!space)
         return;
 
-    auto_gchar gchar* ts_str = g_strndup(buf, space - buf);
+    auto_gchar gchar* ts_str = g_strndup(buf, g_diff_to_gsize(space, buf));
     GDateTime* dt = g_date_time_new_from_iso8601(ts_str, NULL);
     if (!dt) {
         log_warning("flatfile: unparsable timestamp in %s at offset %ld: %s",
@@ -771,7 +771,7 @@ _flatfile_get_previous_chat(const gchar* const contact_barejid, const gchar* sta
 
     // For "last N messages": back up from read_to using index
     if (!from_start) {
-        int margin_entries = (MESSAGES_TO_RETRIEVE * 3) / FF_INDEX_STEP + 2;
+        size_t margin_entries = (MESSAGES_TO_RETRIEVE * 3) / FF_INDEX_STEP + 2;
 
         // Find index entry closest to (but before) read_to
         size_t entry_idx = 0;
@@ -781,7 +781,7 @@ _flatfile_get_previous_chat(const gchar* const contact_barejid, const gchar* sta
             entry_idx = i;
         }
 
-        size_t start_entry = entry_idx > (size_t)margin_entries
+        size_t start_entry = entry_idx > margin_entries
                                  ? entry_idx - margin_entries
                                  : 0;
         off_t backed = state->entries[start_entry].byte_offset;