Show profile setup choices on first launch

[trufae]

With no configured profile, Parla opens an empty chat list and tells the user to find the profile menu. Show the existing setup choices directly on the welcome screen: create a profile, import/link one from another device, sign in with an email address, or use an invitation code.

Reuse the same choice list in the Add Profile dialog. Keep the welcome choices available after cancellation and restore them when the last configured profile is removed. Focus the first choice, keep the empty sidebar out of the way across window resizing, and restore the chat list once a profile is ready. Update the onboarding documentation and remove the old instruction to open the profile menu.

Start the shared event loop with no selected account so first-profile creation and linking receive progress events. This includes the same event-loop startup adjustment as #82, which can be merged independently.

Validation:

• Native Linux builds pass with accessibility enabled and disabled, warnings treated as errors, and webxdc=false.
• All 14 existing Meson test suites pass.
• git diff --check passes.
• Interactive GTK layout and device-transfer checks were not run: this environment prevents Xvfb from opening display sockets.

Fixes #25.