settings: add nav visibility prefs and per-user Strava credentials

- user_prefs table in db.py with get/set helpers
- GET/PUT /api/me/prefs endpoints for bulk pref management
- GET/PUT/DELETE /api/me/strava-credentials; PUT preserves existing
  secret when client_secret field is left blank
- _strava_creds() helper resolves per-user → instance fallback across
  all five Strava endpoints
- Settings page: Navigation card (hide Feed/Community/About toggles)
  and Strava credentials card
- Base.astro: ids on feed/community/about nav links; applies
  nav_hide_* prefs after login

bincio/serve/db.py

@@ -59,9 +59,17 @@ CREATE TABLE IF NOT EXISTS settings (
     value TEXT NOT NULL
 );
 
+CREATE TABLE IF NOT EXISTS user_prefs (
+    handle  TEXT NOT NULL REFERENCES users(handle) ON DELETE CASCADE,
+    key     TEXT NOT NULL,
+    value   TEXT NOT NULL,
+    PRIMARY KEY (handle, key)
+);
+
 CREATE INDEX IF NOT EXISTS sessions_handle ON sessions(handle);
 CREATE INDEX IF NOT EXISTS invites_created_by ON invites(created_by);
 CREATE INDEX IF NOT EXISTS reset_codes_handle ON reset_codes(handle);
+CREATE INDEX IF NOT EXISTS user_prefs_handle ON user_prefs(handle);
 """
 
 _SESSION_DAYS = 30
@@ -361,6 +369,36 @@ def create_reset_code(db: sqlite3.Connection, handle: str, created_by: str) -> s
     return code
 
 
+# ── User preferences ─────────────────────────────────────────────────────────
+
+def get_user_prefs(db: sqlite3.Connection, handle: str) -> dict[str, str]:
+    """Return all preferences for a user as a plain dict."""
+    rows = db.execute(
+        "SELECT key, value FROM user_prefs WHERE handle = ?", (handle,)
+    ).fetchall()
+    return {r["key"]: r["value"] for r in rows}
+
+
+def set_user_pref(db: sqlite3.Connection, handle: str, key: str, value: str) -> None:
+    db.execute(
+        "INSERT INTO user_prefs (handle, key, value) VALUES (?, ?, ?) "
+        "ON CONFLICT(handle, key) DO UPDATE SET value = excluded.value",
+        (handle, key, value),
+    )
+    db.commit()
+
+
+def set_user_prefs(db: sqlite3.Connection, handle: str, prefs: dict[str, str]) -> None:
+    """Bulk-upsert multiple preferences for a user."""
+    for key, value in prefs.items():
+        db.execute(
+            "INSERT INTO user_prefs (handle, key, value) VALUES (?, ?, ?) "
+            "ON CONFLICT(handle, key) DO UPDATE SET value = excluded.value",
+            (handle, key, value),
+        )
+    db.commit()
+
+
 def use_reset_code(db: sqlite3.Connection, code: str, handle: str) -> bool:
     """Validate a reset code for the given handle and mark it used.