Fix progress when linking the first profile

[trufae]

When Parla has no configured profile, Window.try_connect() creates the shared event handler but never starts its event loop. Linking the first device still runs get_backup, while the receiving UI stays at “Connecting… 0%” because it never consumes the core's ImexProgress events.

Start the existing event loop after connection setup regardless of whether a profile is configured. The existing import dialog then receives and displays progress normally. This also restores ConfigureProgress updates during first-profile creation.

Add a small integration check that launches the real app against an empty fake RPC account store under Xvfb. It verifies that the app polls and consumes both import and configuration progress events for an unselected account, then leaves the next long poll pending. The test uses temporary configuration/data directories and runs in the existing Linux CI job.

Validation:

• Native Linux builds pass with accessibility enabled and disabled, warnings treated as errors, and webxdc=false.
• All 14 existing Meson test suites pass.
• The test backend's handshake, empty account response, progress payloads and pending long poll were verified locally; workflow YAML and git diff --check pass.
• Both Linux CI builds passed, including the real-app startup regression. Other platform jobs may still be running.
• A physical two-device transfer has not been exercised.

Fixes #80.