Split the UI crate into modules

[trypsynth]

What changed

rabbit-ui-wxdragon was two files: lib.rs (6114 lines) and wx_app.rs (6922 lines). Both are now split by topic. No behavior change: this is a move of code, not a rewrite.

Wizard data layer (lib.rs to wizard/)

lib.rs is now 17 lines and only declares the modules and run_gui. The wizard logic moved into wizard.rs plus:

File	What lives there
model.rs	the page data structs and the install request/outcome types
bootstrap.rs	detection, plan build, first WizardModel
text.rs	localized labels and wx mnemonic encoding
target.rs	target page rows, including the custom portable folder
packages.rs	package rows, selection, checkbox toggles
configuration.rs	configuration steps and their package dependencies
request.rs	page selections to WizardInstallRequest
review.rs	review page lines and manual step notes
install.rs	runs the install and reports progress
self_update.rs	update check, apply, and their summaries
outcome.rs	outcome reports and error summaries
summary.rs	SetupReport to done page text
labels.rs	labels shared by the row builders and the run summary

The 2446-line test module became wizard/tests/ with one file per area and shared fixtures in tests/support.rs. All 54 tests kept, all pass.

wx shell (wx_app.rs to wx_app/)

wx_app.rs keeps run and the step constants (1079 lines). The rest moved to:

File	What lives there
globals.rs	the process wide localizer, frame, post install hook, version check dispatcher
close_guard.rs	what closing the window means while an install runs
self_update_ui.rs	status line, prompt with release notes, progress window
progress_ui.rs	ProgressEvent to gauge, status line, log
packages_page.rs	shared aliases plus the per platform module wiring
packages_page/native.rs	the Windows TreeCtrl page
packages_page/native_tree_checkboxes.rs	the Win32 TVS_CHECKBOXES helpers
packages_page/dataview.rs	the non Windows DataViewCtrl page
pages.rs	every build_*_page and add_pages
version_check.rs	the version check page and its worker threads
widgets.rs	WizardWidgets plus the small readers, sync helpers, and update_navigation
shell.rs	open a URL or folder, launch REAPER, relaunch with a locale

The two can_launch_reaper_path / planned_reaper_launch_path_for_target tests moved with their code into widgets.rs.

How it was checked

• Code content was compared before and after with whitespace and imports normalized away. The only differences are rustfmt trailing commas, plus two attributes that are now obsolete (#[allow(unused_imports)] on an import that no longer exists, and the mod native_tree_checkboxes wrapper).
• cargo fmt --all --check, cargo clippy --workspace --all-targets, cargo clippy -p rabbit-ui-wxdragon --features gui --all-targets, and cargo test --workspace are clean.
• cargo test -p rabbit-ui-wxdragon --features gui runs 56 tests, all pass.
• Items that only needed to be visible inside the crate are pub(crate), so the public API is still run_gui plus the wizard types.

Not done

run in wx_app.rs is still about 1000 lines. Its event handler blocks each capture a dozen Rc/Arc clones from the surrounding scope, so pulling them out means either 20 parameter functions or first bundling that state into a struct. That is a design change rather than a move, so it is left for a follow up.