feat: add ctx.prompt.form() API for multi-field form dialogs

Add PromptFormRequest and PromptFormResponse types to enable plugins to
display forms with multiple input fields. Implement the form() method in
the prompt context and wire up frontend event handling to show and collect
form responses from users.

crates/yaak-plugins/src/events.rs

@@ -157,6 +157,9 @@ pub enum InternalEventPayload {
     PromptTextRequest(PromptTextRequest),
     PromptTextResponse(PromptTextResponse),
 
+    PromptFormRequest(PromptFormRequest),
+    PromptFormResponse(PromptFormResponse),
+
     WindowInfoRequest(WindowInfoRequest),
     WindowInfoResponse(WindowInfoResponse),
 
@@ -571,6 +574,28 @@ pub struct PromptTextResponse {
     pub value: Option<String>,
 }
 
+#[derive(Debug, Clone, Default, Serialize, Deserialize, TS)]
+#[serde(default, rename_all = "camelCase")]
+#[ts(export, export_to = "gen_events.ts")]
+pub struct PromptFormRequest {
+    pub id: String,
+    pub title: String,
+    #[ts(optional)]
+    pub description: Option<String>,
+    pub inputs: Vec<FormInput>,
+    #[ts(optional)]
+    pub confirm_text: Option<String>,
+    #[ts(optional)]
+    pub cancel_text: Option<String>,
+}
+
+#[derive(Debug, Clone, Default, Serialize, Deserialize, TS)]
+#[serde(default, rename_all = "camelCase")]
+#[ts(export, export_to = "gen_events.ts")]
+pub struct PromptFormResponse {
+    pub values: Option<HashMap<String, JsonPrimitive>>,
+}
+
 #[derive(Debug, Clone, Default, Serialize, Deserialize, TS)]
 #[serde(default, rename_all = "camelCase")]
 #[ts(export, export_to = "gen_events.ts")]