feat: Add Flush Cache tool page to web UI

Adds a /flush-cache web page with a button to clear all cached Google
Sheets data and reset refresh timers. Link added to Tools nav across
all templates.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app.py

@@ -7,7 +7,7 @@ import os
 import io
 import qrcode
 import logging
-from flask import Flask, render_template, g, send_file, request, jsonify
+from flask import Flask, render_template, g, send_file, request
 
 # Configure logging, allowing override via LOG_LEVEL environment variable
 log_level = os.environ.get("LOG_LEVEL", "INFO").upper()
@@ -117,10 +117,12 @@ def index():
     # Redirect root to /adults for convenience while there are no other apps
     return '<meta http-equiv="refresh" content="0; url=/adults" />'
 
-@app.route("/flush-cache", methods=["POST"])
+@app.route("/flush-cache", methods=["GET", "POST"])
 def flush_cache_endpoint():
+    if request.method == "GET":
+        return render_template("flush-cache.html")
     deleted = flush_cache()
-    return jsonify({"status": "ok", "deleted_files": deleted})
+    return render_template("flush-cache.html", flushed=True, deleted=deleted)
 
 @app.route("/sync-bank")
 def sync_bank():