core: implement /theme save and /theme del

Add two complementary subcommands:

  /theme save <name> [-full]: writes a user theme file at
    ${weechat_config_dir}/themes/<name>.theme containing the current
    themable options. By default only options whose value differs from
    their default (config_file_option_has_changed) are written, which
    keeps the file small and focused. Pass "-full" to write every
    themable option (matches the format used by automatic backups).

    Name validation: refuses any name matching a built-in theme (those
    are reserved for in-memory registrations) and any name starting
    with "backup-" (reserved for /theme apply backups). Both checks
    print an error and abort without writing.

  /theme del <name>: removes ${weechat_config_dir}/themes/<name>.theme
    via unlink. Refuses to delete a name registered as a built-in
    theme (a built-in has no file on disk to delete, even if the user
    has a shadowing file of the same name they cannot remove it this
    way; they can rename or delete it manually).

The full-snapshot writer used by /theme apply backups is refactored
into theme_write_file (name, description, diff_only). It is reused
by theme_make_backup (diff_only=0) and theme_save (diff_only inverted
from the user's -full flag).

src/core/core-theme.c

@@ -312,12 +312,16 @@ theme_make_backup_name (void)
 }
 
 /*
- * Write a full snapshot of every themable option to a .theme file at
+ * Write a snapshot of themable options to a .theme file at
  * "<weechat_config_dir>/themes/<name>.theme".
  *
  * The themes directory is created if missing. The file contains an
  * [info] section (name, description, date, weechat version) followed by
- * an [options] section listing every themable option's current value.
+ * an [options] section.
+ *
+ * If "diff_only" is non-zero, only options whose value differs from
+ * their default (config_file_option_has_changed) are written. If zero,
+ * every themable option is written (full snapshot).
  *
  * Return:
  *   1: success
@@ -325,7 +329,7 @@ theme_make_backup_name (void)
  */
 
 int
-theme_write_file_full (const char *name, const char *description)
+theme_write_file (const char *name, const char *description, int diff_only)
 {
     char *path, *dir, *value, *now;
     FILE *file;
@@ -374,6 +378,8 @@ theme_write_file_full (const char *name, const char *description)
             {
                 if (!ptr_option->themable)
                     continue;
+                if (diff_only && !config_file_option_has_changed (ptr_option))
+                    continue;
                 value = config_file_option_value_to_string (
                     ptr_option, 0, 0, 1);
                 fprintf (file, "%s.%s.%s = %s\n",
@@ -405,9 +411,10 @@ theme_make_backup (void)
     name = theme_make_backup_name ();
     if (!name)
         return NULL;
-    if (!theme_write_file_full (
+    if (!theme_write_file (
             name,
-            _("Automatic backup written before /theme apply")))
+            _("Automatic backup written before /theme apply"),
+            0))  /* full snapshot: backups must round-trip exactly */
     {
         free (name);
         return NULL;
@@ -785,6 +792,105 @@ theme_apply (const char *name)
     return WEECHAT_RC_OK;
 }
 
+/*
+ * Save the current themable options to a user theme file.
+ *
+ * Refuse names that match a built-in theme (registered via API) or
+ * that start with "backup-" (reserved for automatic backups). If
+ * "full" is non-zero, every themable option is written; otherwise
+ * only options whose value differs from their default are written.
+ *
+ * Return WEECHAT_RC_OK on success, WEECHAT_RC_ERROR on validation or
+ * I/O failure.
+ */
+
+int
+theme_save (const char *name, int full)
+{
+    if (!name || !name[0])
+        return WEECHAT_RC_ERROR;
+
+    if (strncmp (name, "backup-", 7) == 0)
+    {
+        gui_chat_printf (
+            NULL,
+            _("%sName \"%s\" is reserved for automatic backups"),
+            gui_chat_prefix[GUI_CHAT_PREFIX_ERROR],
+            name);
+        return WEECHAT_RC_ERROR;
+    }
+
+    if (theme_search (name))
+    {
+        gui_chat_printf (
+            NULL,
+            _("%sName \"%s\" is reserved for a built-in theme"),
+            gui_chat_prefix[GUI_CHAT_PREFIX_ERROR],
+            name);
+        return WEECHAT_RC_ERROR;
+    }
+
+    if (!theme_write_file (name, NULL, (full) ? 0 : 1))
+    {
+        gui_chat_printf (NULL,
+                         _("%sFailed to save theme \"%s\""),
+                         gui_chat_prefix[GUI_CHAT_PREFIX_ERROR],
+                         name);
+        return WEECHAT_RC_ERROR;
+    }
+
+    gui_chat_printf (NULL,
+                     _("Theme saved: %s"),
+                     name);
+    return WEECHAT_RC_OK;
+}
+
+/*
+ * Delete a user theme file.
+ *
+ * Refuse names registered as built-in themes (they have no file).
+ * Return WEECHAT_RC_OK on success, WEECHAT_RC_ERROR otherwise.
+ */
+
+int
+theme_delete (const char *name)
+{
+    char *path;
+
+    if (!name || !name[0])
+        return WEECHAT_RC_ERROR;
+
+    if (theme_search (name))
+    {
+        gui_chat_printf (
+            NULL,
+            _("%sCannot delete built-in theme \"%s\""),
+            gui_chat_prefix[GUI_CHAT_PREFIX_ERROR],
+            name);
+        return WEECHAT_RC_ERROR;
+    }
+
+    path = theme_user_file_path (name);
+    if (!path)
+        return WEECHAT_RC_ERROR;
+
+    if (unlink (path) != 0)
+    {
+        gui_chat_printf (NULL,
+                         _("%sFailed to delete theme \"%s\""),
+                         gui_chat_prefix[GUI_CHAT_PREFIX_ERROR],
+                         name);
+        free (path);
+        return WEECHAT_RC_ERROR;
+    }
+
+    gui_chat_printf (NULL,
+                     _("Theme deleted: %s"),
+                     name);
+    free (path);
+    return WEECHAT_RC_OK;
+}
+
 /*
  * Initialize the theme subsystem.
  *