Title bar: inactive tabs without outlines, File menu, Help on the right (#1277)

* Title bar: inactive tabs without outlines, File menu, Help on the right

Inactive tabs go back to the plain bar colour (the Home tab's readable
hover stays). The separate Home button and theme toggle are gone: Home is
a tab and the theme is in Preferences. The folder icon becomes a File text
menu, Save moves into it (Ctrl+S unchanged), and Help sits on the right
next to Report a bug. Tabs start right after File, Undo and Redo.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* Title bar: File button hover uses the app bar hover token

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

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Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

crates/ui-egui/src/colour_vision_tests.rs

@@ -373,7 +373,6 @@ const NO_MEANING: &[(&str, &str)] = &[
     ("overlay", "a chip's ground over the view"),
     ("app_bar_text", "text"),
     ("app_bar_hover", "a hovered tab, translucent white"),
-    ("app_bar_tab_edge", "an inactive tab's outline, translucent white"),
     ("timeline_marker", "the rollback marker, alone"),
     ("field", "a field's ground"),
     // Highlights: shades of the accent blue over what they mark, by design close to each other.