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_sync_client/src/lib.rs

@@ -27,11 +27,13 @@
 pub mod auth;
 pub mod client;
 pub mod device;
+pub mod email_otp;
 pub mod error;
 pub mod snapshot;
 
 pub use auth::{BearerToken, BearerTokenStore};
 pub use client::{ApiClientConfig, SyncApiClient};
 pub use device::{DeviceIdentity, DeviceListResponse, DeviceRecord, DeviceRegistration};
+pub use email_otp::{send_email_otp, verify_email_otp};
 pub use error::SyncClientError;
 pub use snapshot::{SnapshotDownload, SnapshotPayload, SnapshotUploadRequest};