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

backend/passwords.py

@@ -0,0 +1,83 @@
+"""Password hashing (ADR-0001): PBKDF2-SHA256 over an HMAC pepper, plus loading that pepper."""
+
+import base64
+import functools
+import hashlib
+import hmac
+import logging
+import os
+import secrets
+from pathlib import Path
+
+log = logging.getLogger(__name__)
+
+ITERATIONS = 600_000
+SALT_BYTES = 16
+MIN_PEPPER_CHARS = 32
+
+
+def hash_password(
+    password: str,
+    pepper: bytes,
+    *,
+    salt: bytes | None = None,
+    iterations: int = ITERATIONS,
+) -> str:
+    salt = secrets.token_bytes(SALT_BYTES) if salt is None else salt
+    derived = _derive(password, pepper, salt, iterations)
+    return f"pbkdf2_sha256${iterations}${_b64(salt)}${_b64(derived)}"
+
+
+def verify_password(password: str, stored: str, pepper: bytes) -> bool:
+    _, iterations, salt, expected = stored.split("$")
+    derived = _derive(password, pepper, base64.b64decode(salt), int(iterations))
+    return hmac.compare_digest(derived, base64.b64decode(expected))
+
+
+def needs_rehash(stored: str) -> bool:
+    return int(stored.split("$")[1]) < ITERATIONS
+
+
+def _derive(password: str, pepper: bytes, salt: bytes, iterations: int) -> bytes:
+    peppered = hmac.new(pepper, password.encode("utf-8"), hashlib.sha256).digest()
+    return hashlib.pbkdf2_hmac("sha256", peppered, salt, iterations)
+
+
+def _b64(raw: bytes) -> str:
+    return base64.b64encode(raw).decode()
+
+
+@functools.cache
+def dummy_hash() -> str:
+    # Verified against for unknown usernames so their response time matches a real account.
+    return hash_password(secrets.token_urlsafe(16), b"\0" * MIN_PEPPER_CHARS)
+
+
+def load_pepper(pepper_file: Path) -> bytes:
+    from_env = os.environ.get("PASSWORD_PEPPER")
+    if from_env is not None:
+        if len(from_env) < MIN_PEPPER_CHARS:
+            raise RuntimeError(
+                f"PASSWORD_PEPPER must be at least {MIN_PEPPER_CHARS} characters. "
+                'Generate one with: python -c "import secrets; print(secrets.token_hex(32))"'
+            )
+        return from_env.encode()
+    try:
+        fd = os.open(pepper_file, os.O_WRONLY | os.O_CREAT | os.O_EXCL, 0o600)
+    except FileExistsError:
+        pass
+    else:
+        with os.fdopen(fd, "w") as f:
+            f.write(secrets.token_hex(32))
+    log.warning(
+        "PASSWORD_PEPPER is not set; using %s. Development only: production must supply the pepper "
+        "from a secrets manager.",
+        pepper_file,
+    )
+    pepper = pepper_file.read_text().strip()
+    if len(pepper) < MIN_PEPPER_CHARS:
+        raise RuntimeError(
+            f"{pepper_file} holds fewer than {MIN_PEPPER_CHARS} characters. Set PASSWORD_PEPPER, or delete "
+            "the file to regenerate it (existing passwords will stop verifying)."
+        )
+    return pepper.encode()