feat: auth by default, theme toggle, CoderPad startup, review fixes

- Require a session on every route; public routes opt out with @allow_anonymous
- Split password hashing and pepper loading into passwords.py
- Add a system/light/dark theme toggle backed by light-dark() colors
- Ignore stale 401s from an earlier session, PATCH only changed book fields,
  and block overlapping journal-entry saves
- Add bin/start and CoderPad Vite server settings for the pad's start/restart
- Rewrite README as a mise onboarding guide; expand .gitignore
- Include review-round fixes and tests

index.html

@@ -4,6 +4,12 @@
     <meta charset="UTF-8" />
     <meta name="viewport" content="width=device-width, initial-scale=1.0" />
     <title>My Library</title>
+    <script>
+      try {
+        const theme = localStorage.getItem("theme");
+        if (theme === "light" || theme === "dark") document.documentElement.dataset.theme = theme;
+      } catch {}
+    </script>
   </head>
   <body>
     <div id="root"></div>