fix(temas/color): guard role override color input against empty value

The per-role override <input type="color"> took value={r.solidHex} directly; an
expression-driven color input warns ("specified value '' does not conform") if it
ever receives an empty string. r.solidHex is always a valid hex in practice, but
guard it (`|| '#000000'`) so the input can never receive '' — belt-and-suspenders
for a warning seen during live HMR editing.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

web/routes/temas/color/+page.svelte

@@ -291,7 +291,7 @@
 						<span class="role-pin">
 							<input
 								type="color"
-								value={r.solidHex}
+								value={r.solidHex || '#000000'}
 								oninput={(e) => (overrides[r.role] = e.currentTarget.value)}
 								title="sobrescribir {r.role}"
 							/>