chore: drop non-code junk (pi-mono-reference, runtime state, root duplicates)

Remove files that should never have been tracked:

- pi-mono-reference/: 688-file copy of an upstream monorepo, zero imports,
  used only as offline human reference
- build/electron-builder.runtime.json: environment-specific build artifact
  with hardcoded local paths
- state/cancel-signal-state.json: runtime state written by the app
- prompt.txt, logo.svg (root): exact duplicates of the canonical copies
  under .planning/archive/ and assets/

Add state/ to .gitignore so future runtime state stays out.

pi-mono-reference/packages/web-ui/src/dialogs/ApiKeyPromptDialog.ts

@@ -1,75 +0,0 @@
-import { customElement, state } from "lit/decorators.js";
-import "../components/ProviderKeyInput.js";
-import { DialogContent, DialogHeader } from "@mariozechner/mini-lit/dist/Dialog.js";
-import { DialogBase } from "@mariozechner/mini-lit/dist/DialogBase.js";
-import { html } from "lit";
-import { getAppStorage } from "../storage/app-storage.js";
-import { i18n } from "../utils/i18n.js";
-
-@customElement("api-key-prompt-dialog")
-export class ApiKeyPromptDialog extends DialogBase {
-	@state() private provider = "";
-
-	private resolvePromise?: (success: boolean) => void;
-	private unsubscribe?: () => void;
-
-	protected modalWidth = "min(500px, 90vw)";
-	protected modalHeight = "auto";
-
-	static async prompt(provider: string): Promise<boolean> {
-		const dialog = new ApiKeyPromptDialog();
-		dialog.provider = provider;
-		dialog.open();
-
-		return new Promise((resolve) => {
-			dialog.resolvePromise = resolve;
-		});
-	}
-
-	override async connectedCallback() {
-		super.connectedCallback();
-
-		// Poll for key existence - when key is added, resolve and close
-		const checkInterval = setInterval(async () => {
-			const hasKey = !!(await getAppStorage().providerKeys.get(this.provider));
-			if (hasKey) {
-				clearInterval(checkInterval);
-				if (this.resolvePromise) {
-					this.resolvePromise(true);
-					this.resolvePromise = undefined;
-				}
-				this.close();
-			}
-		}, 500);
-
-		this.unsubscribe = () => clearInterval(checkInterval);
-	}
-
-	override disconnectedCallback() {
-		super.disconnectedCallback();
-		if (this.unsubscribe) {
-			this.unsubscribe();
-			this.unsubscribe = undefined;
-		}
-	}
-
-	override close() {
-		super.close();
-		if (this.resolvePromise) {
-			this.resolvePromise(false);
-		}
-	}
-
-	protected override renderContent() {
-		return html`
-			${DialogContent({
-				children: html`
-					${DialogHeader({
-						title: i18n("API Key Required"),
-					})}
-					<provider-key-input .provider=${this.provider}></provider-key-input>
-				`,
-			})}
-		`;
-	}
-}