build: Pikaur-safe hardening flags, opt-in sanitizers, -Wsign-compare

Re-introduce the build hardening that 5459e78e8 attempted, in a form that
coexists with Arch/Pikaur builds (the conflict that forced its revert in
0722dc9e3).

configure.ac:
- Add -D_FORTIFY_SOURCE=2 only when neither $CFLAGS nor $CPPFLAGS already
  define _FORTIFY_SOURCE (makepkg injects its own) and not under --coverage.
- Route glib/gio CFLAGS through -isystem so their macros don't trip our -W set.
- Add --enable-sanitizers (ASan + UBSan; unsigned-integer-overflow probed,
  skipped on gcc) for a dedicated CI job, off by default.
- Add --enable-hardening to gate -Wnull-dereference (false positives under
  -O2 on some toolchains), off by default.
- Promote -Wsign-compare from -Wno- to an active probed warning.

Also fix the sign-conversion sites that -Wsign-compare/-Wconversion cleanup
surfaced as safe (pointer-diff to gsize via new g_diff_to_gsize helper,
non-negative int casts), and validate the /vcard remove index through
strtoi_range so a negative argument can no longer become a huge unsigned
index.

Refs #112

src/command/cmd_funcs.c

@@ -10063,8 +10063,14 @@ cmd_vcard_remove(ProfWin* window, const char* const command, gchar** args)
     }
 
     if (args[1]) {
-        vcard_user_remove_element(atoi(args[1]));
-        cons_show("Removed element at index %d", atoi(args[1]));
+        int index;
+        auto_gchar gchar* err_msg = NULL;
+        if (!strtoi_range(args[1], &index, 0, INT_MAX, &err_msg)) {
+            cons_show("%s", err_msg);
+            return TRUE;
+        }
+        vcard_user_remove_element((unsigned int)index);
+        cons_show("Removed element at index %d", index);
         vcardwin_update();
     } else {
         cons_bad_cmd_usage(command);