Add email + OTP sign-in flow for cloud sync

Replace the "drop a session token in a file" workflow with a real
Chrome-style email login. The Cloudflare worker already had Better
Auth's `email-otp` plugin wired into `SEND_EMAIL`; this commit
builds the renderer-side counterpart.

Worker side:
- Move the OTP sender from `auth@elydora.com` to `browser@elydora.com`
  (wrangler.toml `allowed_sender_addresses` + better_auth.ts
  `EMAIL_OTP_FROM_ADDRESS`). Worker must be redeployed to pick this up.

Client side (`ely_sync_client::email_otp`):
- `send_email_otp(config, email)` POSTs `/api/auth/email-otp/send-verification-otp`
  with `{ email, type: "sign-in" }`.
- `verify_email_otp(config, email, otp)` POSTs `/api/auth/sign-in/email-otp`,
  reads the Better Auth session token from the JSON body's `token` field
  with the `Set-Cookie: better-auth.session_token=…` header as the
  documented fallback channel, and returns it as a `BearerToken`.

Shell side (`shell/auth.rs` + `shell/internal_pages/sync.rs`):
- New `AuthFlowPhase` (Idle / SendingCode / AwaitingOtp / Verifying /
  Error) tracks the in-flight form. Two off-thread workers run the
  HTTP exchanges so the GPUI render loop never blocks.
- Successful verify saves the bearer via `SyncEngine::install_bearer`
  and triggers an immediate snapshot upload, so the user is signed in
  + initial-synced in one click.
- Sync settings page replaces the bare "Sync now" button row with an
  account card: when SignedOut → email field + Send code → OTP field
  + Verify / Resend; when signed in → an account chip + Sign out.
- `trigger_cloud_sync_upload` no longer takes a `Context` param so
  the post-auth path can fire it from the inbox-drain pass without
  needing a window context.

crates/ely_app/src/shell/settings_actions.rs

@@ -246,7 +246,7 @@ impl ElyShell {
     /// (the UI thread never blocks on the network), and the worker
     /// reports back through the shell's `sync_inbox` so the sync page
     /// reflects the new state without waiting for a manual refresh.
-    pub(super) fn trigger_cloud_sync_upload(&mut self, _cx: &mut Context<Self>) {
+    pub(crate) fn trigger_cloud_sync_upload(&mut self) {
         let ShellState::Ready(core) = &self.state else {
             return;
         };
@@ -318,7 +318,11 @@ fn run_sync_upload(
     bytes: Vec<u8>,
     inbox: std::sync::mpsc::Sender<super::SyncStateUpdate>,
 ) {
-    let mut engine = match SyncEngine::for_profile_dir(&profile_dir, device_name, sync_platform()) {
+    let mut engine = match SyncEngine::for_profile_dir(
+        &profile_dir,
+        device_name,
+        super::sync_platform_label(),
+    ) {
         Ok(engine) => engine,
         Err(error) => {
             let message = error.to_string();
@@ -364,15 +368,3 @@ fn run_sync_upload(
         }
     }
 }
-
-const fn sync_platform() -> &'static str {
-    if cfg!(target_os = "macos") {
-        "macos"
-    } else if cfg!(target_os = "windows") {
-        "windows"
-    } else if cfg!(target_os = "linux") {
-        "linux"
-    } else {
-        "other"
-    }
-}