Add workspace setting for attachment size limit

[careck]

Description

The core library already has attachment_max_size_bytes() / set_attachment_max_size_bytes() methods on Workspace backed by the workspace_meta table, but these are not exposed to the user through the UI or Tauri commands.

Add a workspace-level setting that allows users to configure the maximum attachment file size per workspace.

Current state

• Workspace::attachment_max_size_bytes() returns Option<u64> from workspace_meta
• Workspace::set_attachment_max_size_bytes(Option<u64>) writes to workspace_meta
• Default is None (unlimited) — should default to 10 MB to match relay bundle limit (Add size limits for inline attachment embedding #184)
• No Tauri command or UI exposes this setting

Acceptance criteria

• Tauri commands to get/set the attachment size limit per workspace
• UI control in workspace settings to view and change the limit
• Sensible default (10 MB) when no value is configured
• Validation: reject values ≤ 0 or unreasonably large
• i18n for all new UI strings