Split main.rs: 28k lines, a 1,700-line main() and an 86-field WorkerState

[Sythos]

Closes #206.

Last step of the split of main.rs. The 172 ui.on_* registrations of main() move verbatim into 21 functions in the new crates/cordiale-ui/src/ui_callbacks.rs (radio, connect, security, passkeys, saved profile, navigation, members, channels, directory, DCC and archive, links, views, attach, composer, themes, notifications, four admin areas and the settings screens). Every function is pub(crate) fn register_<area>_callbacks(ui: &AppWindow, worker_tx: &mpsc::UnboundedSender<WorkerCommand>) and is called as register_<area>_callbacks(&ui, &worker_tx); in the same order as the original statements. main() goes from 1,725 to 168 lines and main.rs from 12,391 to 10,836. What stays inline: the setup and the settings-driven UI state, the worker channel and thread, the auto_connect block, the foreground timer (a slint::Timer that must live until the end of main()), the cfg-gated key_prompt::install(&ui) and ui.run() with the quit handshake.

Both parameters are only used by shared reference (ui.on_*, ui.as_weak(), ui.window() and worker_tx.clone()), nothing is moved out of main(), and no Slint callback closure captures a reference parameter (they 'static-capture clones made inside the run, exactly as before). Checked by script: every helper body is token-identical to the original statements (11,563 tokens, no normalisation needed); the token stream of main.rs with the runs masked equals the original (64,460 tokens), apart from the mod and use lines and the 21 call statements; every name bound inside a run stays inside it; no ?, return, break, continue or label outside closure bodies; no name collides with another module; the #[test] count is unchanged (305). The CI log should show the same test counts as main (390 for cordiale-ui). Not verified: nothing is compiled before CI, and runtime behaviour is unchanged only by construction (same statements, same order, same ownership).

This closes the series. What landed under #206, each as its own pull request with the test count checked against main:

• tests moved out of main.rs (Move the unit tests of cordiale-ui out of main.rs #218);
• the frame dispatch, slash commands, admin handlers, history, channel and query window handling in their own modules (Move the frame dispatch of cordiale-ui to frames.rs #221, Move the slash commands of cordiale-ui to slash.rs #222, Move the admin handlers of cordiale-ui to admin_handlers.rs #223, Move history, channel and query window handling to their own modules #224);
• the 88 fields of WorkerState grouped into six sub-structs (Group the panel fields of WorkerState into PanelState #225, Group the settings and preferences fields of WorkerState into SettingsState #226, Group the per-network fields of WorkerState into NetworkState #227, Group the transcript and window-content fields of WorkerState into TranscriptState #228, Group the connection and session fields of WorkerState into SessionState #229, Group the window and sidebar fields of WorkerState into WindowState #230);
• 26 run_worker command bodies extracted (Move 26 run_worker command bodies into worker_commands.rs #231);
• and this one.

main.rs went from 28,699 to about 10,800 lines. Still open for later: a further split of the remaining run_worker arms and of the 10,800 lines of main.rs itself, now cheap to do one module at a time.

Branch: enhancement-issue-206-main