feat: add spellcheck highlighting support

Introduce optional spellcheck highlighting in the input window using
the Enchant-2 library.

```
/spellcheck on
/spellcheck lang en_US
```

New theme color `input.misspelled`.

Fixes: https://github.com/profanity-im/profanity/issues/183
Signed-off-by: Michael Vetter <jubalh@iodoru.org>

src/tools/spellcheck.c

@@ -0,0 +1,107 @@
+/*
+ * spellcheck.c
+ * vim: expandtab:ts=4:sts=4:sw=4
+ *
+ * Copyright (C) 2026 Michael Vetter <jubalh@iodoru.org>
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later WITH OpenSSL-exception
+ */
+
+#include "config.h"
+
+#ifdef HAVE_SPELLCHECK
+
+#include <enchant.h>
+#include <string.h>
+#include <glib.h>
+
+#include "tools/spellcheck.h"
+#include "config/preferences.h"
+#include "log.h"
+#include "common.h"
+
+static EnchantBroker* broker = NULL;
+static EnchantDict* dict = NULL;
+static char* current_lang = NULL;
+
+void
+spellcheck_init(void)
+{
+    if (broker) {
+        return;
+    }
+
+    broker = enchant_broker_init();
+    if (!broker) {
+        log_error("Failed to initialize Enchant broker");
+        return;
+    }
+
+    prof_add_shutdown_routine(spellcheck_deinit);
+
+    char* lang = prefs_get_string(PREF_SPELLCHECK_LANG);
+    spellcheck_set_lang(lang);
+    g_free(lang);
+}
+
+void
+spellcheck_deinit(void)
+{
+    if (dict) {
+        enchant_broker_free_dict(broker, dict);
+        dict = NULL;
+    }
+    if (broker) {
+        enchant_broker_free(broker);
+        broker = NULL;
+    }
+    g_free(current_lang);
+    current_lang = NULL;
+}
+
+gboolean
+spellcheck_set_lang(const char* lang)
+{
+    if (!broker || !lang) {
+        return FALSE;
+    }
+
+    if (current_lang && strcmp(current_lang, lang) == 0 && dict) {
+        return TRUE;
+    }
+
+    EnchantDict* new_dict = enchant_broker_request_dict(broker, lang);
+    if (!new_dict) {
+        log_error("Failed to request Enchant dictionary for language: %s", lang);
+        return FALSE;
+    }
+
+    if (dict) {
+        enchant_broker_free_dict(broker, dict);
+    }
+
+    dict = new_dict;
+    g_free(current_lang);
+    current_lang = g_strdup(lang);
+
+    return TRUE;
+}
+
+const char*
+spellcheck_get_lang(void)
+{
+    return current_lang;
+}
+
+gboolean
+spellcheck_is_misspelled(const char* word)
+{
+    if (!dict || !word || word[0] == '\0') {
+        return FALSE;
+    }
+
+    int result = enchant_dict_check(dict, word, strlen(word));
+    return (result != 0);
+}
+
+#endif

src/tools/spellcheck.h

@@ -0,0 +1,52 @@
+/*
+ * spellcheck.h
+ * vim: expandtab:ts=4:sts=4:sw=4
+ *
+ * Copyright (C) 2026 Michael Vetter <jubalh@iodoru.org>
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later WITH OpenSSL-exception
+ */
+
+#ifndef TOOLS_SPELLCHECK_H
+#define TOOLS_SPELLCHECK_H
+
+#include "config.h"
+#include <glib.h>
+
+#ifdef HAVE_SPELLCHECK
+
+void spellcheck_init(void);
+void spellcheck_deinit(void);
+gboolean spellcheck_is_misspelled(const char* word);
+gboolean spellcheck_set_lang(const char* lang);
+const char* spellcheck_get_lang(void);
+
+#else
+
+static inline void
+spellcheck_init(void)
+{
+}
+static inline void
+spellcheck_deinit(void)
+{
+}
+static inline gboolean
+spellcheck_is_misspelled(const char* word)
+{
+    return FALSE;
+}
+static inline gboolean
+spellcheck_set_lang(const char* lang)
+{
+    return FALSE;
+}
+static inline const char*
+spellcheck_get_lang(void)
+{
+    return NULL;
+}
+
+#endif
+
+#endif