Home: axonometric view, fitted

DesignCanvas::set_view() and fit_view() were both written and then never called
from anywhere in the tree. The Design viewport has had no way back to a standard
view since it existed: no key, no button, nothing but orbiting by hand until the
model happens to drift into frame.

That is worse than a missing convenience. A camera left pointing along the bed
plane renders a scene that looks exactly like a failed renderer — geometry
present, nothing visible — and an hour went into blaming the software GL stack
before the real cause turned out to be two uncalled functions.

Home rather than a letter: every letter A-Z is already a Shift+letter tool
shortcut. Home is also the reset-the-view key most users arrive with. The
dispatcher needed no change, it keys on the raw wx keycode. set_view() already
does select_view + zoom_to_volumes, so this is fit and orient in one call.

Doc row added to the View toggles table in docs/design_tab.md.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

docs/design_tab.md

@@ -137,6 +137,7 @@ selected by the mode, not by whether a sketch session is running.
 
 | Key | Action |
 |---|---|
+| `Home` | Axonometric view, fitted to the model |
 | `P` | Origin planes on/off |
 | `A` | World axes on/off |
 | `X` | Section view on/off |