feat(gui): explain Run Diagnostic + offer to launch the game — 0.13.0

The recording banner gave no guidance, so it wasn't clear what to do after
clicking Run Diagnostic.

- Start dialog now spells out the flow: play the game, reproduce the crash, then
  Finish & analyze (data survives a hard freeze + reboot), with "Launch game &
  start" (steam.launch_game via steam:// appid URL) or "Start without launching".
- Recording banner now states the next step, not just a sample count.
- steam.launch_game(appid): best-effort Steam launch (steam / xdg-open).
- Fix: escape "&" in button labels (Qt mnemonic) so "Finish & analyze" shows
  correctly instead of "Finish _analyze".

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

CHANGELOG.md

@@ -5,6 +5,17 @@ All notable changes to RigDoctor are recorded here. Format follows
 (`MAJOR.MINOR.PATCH`, pre-1.0). `__version__` and `pyproject.toml` must match the git
 release tag (so the auto-updater, D18, can compare versions).
 
+## [0.13.0] - 2026-05-22
+### Added
+- **Run Diagnostic now explains itself and can launch the game.** Clicking Run Diagnostic shows
+  what to do — *play the game, reproduce the crash, then Finish & analyze* (and that data
+  survives a hard freeze + reboot) — and offers **Launch game & start** (asks Steam to run it by
+  appid) or **Start without launching**. The recording banner now spells out the next step
+  instead of just showing a sample count.
+### Fixed
+- Button labels containing "&" (e.g. "Finish & analyze") rendered as "Finish _analyze" because
+  Qt treated the "&" as a keyboard mnemonic — now escaped so the ampersand shows literally.
+
 ## [0.12.0] - 2026-05-22
 ### Added
 - **Guided diagnostic in the GUI.** Each game on the **Games** page now has a **Run Diagnostic**