Add Upload/Download over Cellular Toggle to Settings and Sync Page

[chadw-eten]

Fluent translators frequently work in areas where cellular data is limited or expensive. By default, Fluent should not use cellular data for uploads or downloads — but users who choose to allow it should be able to opt in. This ticket adds an off-by-default "Upload/Download over cellular" toggle to the Settings menu as the canonical location, and surfaces the same setting on the Sync page for convenience.

Functional Requirements

Settings Menu — Upload/Download over Cellular

• Add a toggle row to the Settings menu below the Prepare for Offline setting with the label "Upload/Download over cellular" and the description "Use mobile data to upload recordings when WiFi isn't available."
• The toggle is off by default.
• When off, uploads and downloads only initiate or continue over WiFi; if WiFi is lost while either is active, pause automatically.
• When on, uploads and downloads proceed over any available connection.

Sync Page — Upload/Download over Cellular

• Display the same toggle row on the Sync page, below the Sync Now / Pause / Cancel buttons and above the download queue progress section.
• Use the same label and description text as the Settings instance.
• This control mirrors the Settings toggle — changing either one updates the other immediately.

Note: Both controls represent the same underlying setting. They must stay in sync — changing the value on the Sync page is reflected in Settings and vice versa.

Note: #40 (now closed) included an upload-only cellular toggle in the Settings page. This ticket extends that to cover downloads as well and adds a mirrored toggle on the Sync page. A partial implementation may already exist — scope here extends it.