frame: clear target on the target verb when unit is "none"

Retail's SecureActionButton target action treats a "none" unit as "clear
target". Match it: the `target` verb calls ClearTarget() when the resolved
`unit` is "none" (case-insensitive), before the normal cursor/spell
precedence. Mouseover already handles "none" implicitly (it resolves to
GUID 0 = no unit). Docs' target-verb row updated.

Author: brues-code

docs/API.md

@@ -3874,7 +3874,7 @@ modifier/button-qualified, same precedence as `type`):
 
 | Verb | Extra attributes | Action |
 |------|------------------|--------|
-| `target` | — | Targets the `unit`. Respects the engine's default-interaction precedence: with a spell on the cursor it casts on the unit, with an item on the cursor it drops it on the unit, instead of switching target. |
+| `target` | — | Targets the `unit` (or clears the target if `unit` is `"none"`). Respects the engine's default-interaction precedence: with a spell on the cursor it casts on the unit, with an item on the cursor it drops it on the unit, instead of switching target. |
 | `assist` | — | Targets the `unit`'s target. |
 | `focus` | — | Sets the ClassicAPI focus to the `unit`. |
 | `spell` | `spell` | Casts the `spell` on the `unit` via [`C_Spell.CastAtUnit`](#c_spellcastatunitspellidorname-unit) — the unit's GUID goes straight to the cast dispatcher (no target juggling), and ground-target spells land at the unit's feet. |

src/frame/Attributes.cpp

@@ -553,6 +553,11 @@ bool DispatchVerb(void *L, int fi, const char *prefix, const char *suffix,
                   const char *verb, const char *unit) {
     if (EqI(verb, "target")) {
         if (!unit) return false;
+        // `unit="none"` clears the target (retail's SecureActionButton behavior).
+        if (EqI(unit, "none")) {
+            CallGlobal(L, "ClearTarget");
+            return true;
+        }
         // Cursor / pending-spell take precedence, matching the engine's default
         // unit interaction — cast the pending spell / drop the item on the unit
         // instead of switching target. The two predicates are ours (direct C++);