feat: alerts host-filter field with URL query param and notify URL

- Add regex filter input to the Alerts dashboard that filters displayed
  hosts on every keystroke; invalid regex turns the border red
- Initialise the filter from ?filter= in the URL query string
- Change _build_url() to produce /alerts?filter=<hostname> so
  notification links (Pushover, email, Matrix, etc.) land on the
  alerts page pre-filtered to the alerting host

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

hbd/server/http.py

@@ -641,7 +641,7 @@ async def start(
   <meta charset="utf-8">
   <title>Heartbeat — Login</title>
   <style>
-    body {{ font-family: sans-serif; background: #16191d; display: flex;
+    body {{ font-family: sans-serif; background: #f5f5f5; display: flex;
             justify-content: center; align-items: center; height: 100vh; margin: 0; }}
     .box {{ background: #fff; padding: 2em 2.5em; border-radius: 8px;
              box-shadow: 0 2px 12px rgba(0,0,0,.15); min-width: 300px; }}
@@ -657,7 +657,7 @@ async def start(
     .divider {{ text-align: center; margin: 1.2em 0 .8em; color: #999;
                 font-size: .85em; border-top: 1px solid #eee; padding-top: .8em; }}
     .gitea-btn {{ display: flex; align-items: center; justify-content: center;
-                  gap: .5em; width: 100%; padding: .6em; background: #609926;
+                  gap: .5em; width: 100%; padding: .6em; background: #16191d;
                   color: #fff; border-radius: 4px; font-size: 1em; text-align: center;
                   text-decoration: none; box-sizing: border-box; }}
     .gitea-btn:hover {{ background: #4e7d1e; }}