add feed caching, developer settings section, fix deploy toolchain

- cache feed items in localStorage, merge fresh with cached, auto-refresh hourly
- add refresh bar with timestamp, spinning icon during background refresh
- add developer section: backend health check, test push, mass-follow buttons
- add POST /push/test endpoint for test notifications
- activate mise in deploy.sh so bun is always on $PATH

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

deploy.sh

@@ -3,6 +3,9 @@
 # Usage: ./deploy.sh [uberspace-user@host]  (default: serve)
 set -euo pipefail
 
+# activate mise so tool versions (bun, node, etc.) from .mise.toml are on $PATH
+eval "$(mise activate bash --shims)"
+
 REMOTE="${1:-serve}"
 LOCAL_DIR="$(cd "$(dirname "$0")" && pwd)"