Implement audio range requests

This commit enables serving audio files via HTTP Range requests. This is
crucial for allowing HTML5 audio players to seek to specific positions
within an audio file without re-downloading the entire file.

The changes include:
- Modifying `handle_audio` in `server/src/routes/web.rs` to parse
  `Range` headers.
- Implementing `serve_range` to handle partial content responses.
- Adding a `parse_range` helper function.
- Updating tests to verify range request functionality.
- Adding `ACCEPT_RANGES: bytes` header to indicate support for range
  requests.
- Storing recording duration in a sidecar file for faster UI rendering.
- Enhancing the HTML template to support a custom audio player with
  seeking.

server/src/routes/magic.rs

@@ -26,9 +26,7 @@ use crate::auth::AuthenticatedUser;
 use crate::config::Config;
 use crate::error::AppError;
 use crate::magic_link::{MagicLinkStore, PendingMagicLink};
-use crate::web_session::{
-    SessionStore, WebSession, build_session_cookie, generate_token,
-};
+use crate::web_session::{SessionStore, WebSession, build_session_cookie, generate_token};
 
 /// Magic-link tokens only need to survive click → browser-launch → first
 /// request. 60s gives slow systems headroom without lengthening the attack
@@ -58,7 +56,9 @@ pub async fn handle_create(
     body: Option<Json<MagicLinkRequest>>,
 ) -> Result<Json<MagicLinkResponse>, AppError> {
     let req = body.map(|Json(r)| r).unwrap_or_default();
-    let return_to = req.return_to.unwrap_or_else(|| DEFAULT_RETURN_TO.to_owned());
+    let return_to = req
+        .return_to
+        .unwrap_or_else(|| DEFAULT_RETURN_TO.to_owned());
 
     // Open-redirect guard: only accept paths under our own /web/ tree.
     // Reject schemes, hosts, protocol-relative URLs, and traversal.