feat: add fluxer upstream source and self-hosting documentation

- Clone of github.com/fluxerapp/fluxer (official upstream)
- SELF_HOSTING.md: full VM rebuild procedure, architecture overview,
  service reference, step-by-step setup, troubleshooting, seattle reference
- dev/.env.example: all env vars with secrets redacted and generation instructions
- dev/livekit.yaml: LiveKit config template with placeholder keys
- fluxer-seattle/: existing seattle deployment setup scripts

fluxer/packages/hono/src/Flash.tsx

@@ -0,0 +1,51 @@
+/*
+ * Copyright (C) 2026 Fluxer Contributors
+ *
+ * This file is part of Fluxer.
+ *
+ * Fluxer is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * Fluxer is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with Fluxer. If not, see <https://www.gnu.org/licenses/>.
+ */
+
+export interface Flash {
+	message: string;
+	type: 'success' | 'error' | 'info' | 'warning';
+	detail?: string;
+}
+
+export function serializeFlash(flash: Flash): string {
+	return Buffer.from(JSON.stringify(flash)).toString('base64url');
+}
+
+export function parseFlash(cookie: string | undefined): Flash | undefined {
+	if (!cookie) {
+		return undefined;
+	}
+
+	try {
+		const decoded = Buffer.from(cookie, 'base64url').toString('utf-8');
+		const flash = JSON.parse(decoded) as Flash;
+
+		if (
+			typeof flash.message === 'string' &&
+			typeof flash.type === 'string' &&
+			['success', 'error', 'info', 'warning'].includes(flash.type)
+		) {
+			return flash;
+		}
+
+		return undefined;
+	} catch {
+		return undefined;
+	}
+}