refactor: clean narrowing conversions and harden unsigned arithmetic

Cleanup of the conversion-safety warnings exposed by enabling
-Wconversion / -Wsign-compare in the previous commit, plus guard
clauses at the few places where unsigned arithmetic could actually
misbehave.

Build:
- configure.ac drops -Wno-error=conversion and
  -Wno-error=float-conversion. Only -Wno-error=sign-conversion and
  -Wno-error=sign-compare remain, gating the ~230 sign warnings
  inherited from upstream that will be cleaned up in follow-ups.

Type / conversion fixes (no behaviour change):
- Length-like locals in command/cmd_ac.c, command/cmd_funcs.c,
  pgp/gpg.c, tools/autocomplete.c, tools/parser.c and ui/mucwin.c
  switched from int to size_t / glong (matching strlen /
  g_utf8_strlen return type) so we no longer need an (int) cast and
  loop counters / array sizes stay in their natural unsigned domain.
- g_timer_elapsed / GTimeSpan -> int casts in session.c, iq.c,
  core.c, server_events.c, window.c.
- _win_print_wrapped: indent parameter and local curx/maxx switched
  from size_t to int to match _win_indent / getcurx / getmaxx.
- Port casts (int -> unsigned short) at the libstrophe boundary in
  connection.c and session.c, each preceded by
  g_assert(port >= 0 && port <= UINT16_MAX) so the truncation is
  documented at the call-site.
- curl_off_t / fread size_t results cast at usage in http_upload.c,
  http_download.c, omemo/crypto.c.
- strtoul results cast to uint32_t in xmpp/omemo.c and omemo/omemo.c
  where device/prekey IDs are genuinely 32-bit.
- config/color.c: fg/bg/palette indices switched to `short`
  end-to-end (find_col, color_hash, find_closest_col,
  _color_pair_cache_get, cache.pairs), so the ncurses init_pair
  boundary needs at most one (short)i cast for the cache index. Also
  TODO-noted: init_extended_pair is needed for >15-bit palettes.
- xmpp/avatar.c: float arithmetic explicitly casts its int operands.
- tests/functionaltests/proftest.c: read() result handling uses
  size_t for the accumulator, _read_output returns ssize_t, and the
  buffer-shift check happens before space subtraction so the
  expression cannot underflow.

Real-risk guard clauses (the part that actually fixes bugs):
- src/ui/statusbar.c _tabs_width: `end > opened_tabs - 1` rewritten
  as `end < opened_tabs` so opened_tabs == 0 no longer underflows.
- src/ui/statusbar.c _status_bar_draw_extended_tabs: the mirror
  comparison rewritten as `end >= opened_tabs`.
- src/ui/statusbar.c status_bar_draw: replaced
  `MAX(0, getmaxx - (int)_tabs_width)` with an explicit precheck
  before subtraction.
- src/omemo/omemo.c prekey selection: prekey_index is now uint32_t
  and randomized into an unsigned buffer, so modulo with prekeys_len
  cannot yield a negative index for g_list_nth_data.
- src/omemo/crypto.c omemo_decrypt_func: PKCS#5/PKCS#7 unpadding
  reads `plaintext[plaintext_len - 1]`, which would underflow on a
  malformed empty ciphertext and read past the heap buffer. Reject
  plaintext_len == 0 before the padding peek and validate the
  padding byte against the buffer length before the unpad loop.
  Initialise plaintext = NULL so the early `goto out` cannot free
  uninitialised memory.
- src/ui/inputwin.c (4 mbrlen sites) and src/ui/window.c
  _win_print_wrapped: mbrlen() returns 0 for the null wide
  character. The existing checks rejected (size_t)-1 / -2 but
  treated 0 as a valid step, so the surrounding loops would either
  advance by SIZE_MAX (i += ch_len - 1) or spin in place
  (word_pos += 0 forever). Add `|| ch_len == 0` to each guard;
  inside the spell-check word-emission loop also fall back to a
  one-byte advance.
- Defensive `len > 0 ? len - 1 : 0` prechecks at the strlen-based
  g_strndup / loop sites in ui/console.c, plugins/c_api.c and
  plugins/python_plugins.c.

src/ui/mucwin.c

@@ -382,7 +382,7 @@ _mucwin_print_mention(ProfWin* window, const char* const message, const char* co
         auto_gchar gchar* mynick_str = g_utf8_substring(message, pos, pos + mynick_len);
         win_append_highlight(window, THEME_ROOMMENTION_TERM, "%s", mynick_str);
 
-        last_pos = pos + mynick_len;
+        last_pos = pos + (int)mynick_len;
 
         curr = g_slist_next(curr);
     }
@@ -400,8 +400,8 @@ _mucwin_print_mention(ProfWin* window, const char* const message, const char* co
 gint
 _cmp_trigger_weight(gconstpointer a, gconstpointer b)
 {
-    int alen = strlen((char*)a);
-    int blen = strlen((char*)b);
+    size_t alen = strlen((char*)a);
+    size_t blen = strlen((char*)b);
 
     if (alen > blen)
         return -1;
@@ -438,10 +438,10 @@ _mucwin_print_triggers(ProfWin* window, const char* const message, GList* trigge
         }
 
         // found, replace vars if earlier than previous
-        int trigger_pos = trigger_ptr - message_lower;
+        int trigger_pos = (int)(trigger_ptr - message_lower);
         if (first_trigger_pos == -1 || trigger_pos < first_trigger_pos) {
             first_trigger_pos = trigger_pos;
-            first_trigger_len = strlen(trigger_lower);
+            first_trigger_len = (int)strlen(trigger_lower);
         }
 
         curr = g_list_next(curr);