fix(settings/prompts): enable Save on textarea input and preserve empty strings

Use onInput to update custom support prompts per keystroke (no blur needed). In ModesView, avoid trimming during editing and send raw values to backend; preserve empty strings via ?? to ensure immediate dirty state. Closes #5780.

Author: daniel-lxs

webview-ui/src/components/modes/ModesView.tsx

@@ -1110,7 +1110,7 @@ const ModesView = ({ onDone }: ModesViewProps) => {
 									updateCustomMode(visualMode, {
 										...customMode,
 										// Preserve empty string; only treat null/undefined as unset
-										customInstructions: value.trim() ?? undefined,
+										customInstructions: value ?? undefined,
 										source: customMode.source || "global",
 									})
 								} else {
@@ -1313,7 +1313,7 @@ const ModesView = ({ onDone }: ModesViewProps) => {
 							setCustomInstructions(value ?? undefined)
 							vscode.postMessage({
 								type: "customInstructions",
-								text: value.trim() ?? undefined,
+								text: value ?? undefined,
 							})
 						}}
 						rows={4}

webview-ui/src/components/settings/PromptsSettings.tsx

@@ -167,7 +167,7 @@ const PromptsSettings = ({ customSupportPrompts, setCustomSupportPrompts }: Prom
 					<VSCodeTextArea
 						resize="vertical"
 						value={getSupportPromptValue(activeSupportOption)}
-						onChange={(e) => {
+						onInput={(e) => {
 							const value =
 								(e as unknown as CustomEvent)?.detail?.target?.value ??
 								((e as any).target as HTMLTextAreaElement).value