Rewrite backend in Go with JSON API and JS frontend

Reimplements the Flask app as a Go HTTP API (chi + modernc sqlite) with a
minimal vanilla-JS frontend in web/. Endpoints mirror the original Flask
routes but return JSON instead of HTML.

- auth: login/logout/register/sessioninfo/headerbar with crypto/rand tokens
- entry: paginated feed (single JOIN), create with image scaling
- user: profile, userinfo; delete still a stub
- requireAuth middleware passes uid via context
- notes/api.md documents the API and schema

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

web/css/app.css

@@ -0,0 +1,75 @@
+:root {
+  --bg: #15171a;
+  --card: #1e2126;
+  --fg: #e8e8e8;
+  --muted: #9aa0a6;
+  --accent: #4a9eff;
+}
+
+* { box-sizing: border-box; }
+
+body {
+  margin: 0;
+  font-family: system-ui, sans-serif;
+  background: var(--bg);
+  color: var(--fg);
+}
+
+header {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  padding: 1rem 1.5rem;
+  border-bottom: 1px solid #2a2e35;
+  position: sticky;
+  top: 0;
+  background: var(--bg);
+}
+
+header h1 { margin: 0; font-size: 1.4rem; }
+
+main {
+  max-width: 600px;
+  margin: 0 auto;
+  padding: 1rem;
+}
+
+section { margin-bottom: 1.5rem; }
+
+input, textarea, button {
+  font: inherit;
+  border-radius: 6px;
+  border: 1px solid #2a2e35;
+  background: var(--card);
+  color: var(--fg);
+  padding: 0.6rem 0.8rem;
+}
+
+input, textarea { width: 100%; margin-bottom: 0.6rem; }
+textarea { min-height: 80px; resize: vertical; }
+
+button {
+  cursor: pointer;
+  background: var(--accent);
+  border-color: var(--accent);
+  color: #fff;
+  width: auto;
+}
+button:hover { opacity: 0.9; }
+
+.entry {
+  background: var(--card);
+  border: 1px solid #2a2e35;
+  border-radius: 8px;
+  padding: 1rem;
+  margin-bottom: 1rem;
+}
+
+.entry .meta { color: var(--muted); font-size: 0.85rem; margin-bottom: 0.5rem; }
+.entry .content { white-space: pre-wrap; word-wrap: break-word; }
+.entry img { max-width: 100%; border-radius: 6px; margin-top: 0.6rem; }
+
+.entry a { color: var(--accent); }
+
+.msg { color: #ff6b6b; font-size: 0.9rem; min-height: 1.2em; }
+.end { text-align: center; color: var(--muted); padding: 2rem 0; }