feat(app): sidebar display preferences, a shared menu engine, and a colour pass

Sidebar polish round.

Menus. `dropdown-menu` and `context-menu` were two copies of one engine that
differed only in how the trigger fires — identical from `computePosition` down —
and neither could nest. Both are now thin wrappers over
`components/ui/menu/`, which owns anchoring, a pure page-stack navigation model,
and one presentation fork: flyout on pointer, pushed pages in a sheet on compact.
All 31 call sites are unchanged. Items got a rounded inset fill, a visible
separator, and `selected` (a check) split from `active` (a fill).

Sidebar display preferences. The flat wall of four sections becomes one row per
decision with its value, and per-feature toggles over what a workspace row shows:
host, pull request, checks, scripts. Each is independent — a ticked toggle that
draws nothing is a lie. `display-preferences/` hides that grouping lives in
zustand while title source and row items are app settings.

Timestamps. "Last activity" now ticks itself. The label rounds to the minute and
never shows seconds; past a week it becomes a date. The timer lives in the leaf
`<Text>`, at most one per resolution tier shared by every row, and a tick only
re-renders when the string actually changed — so a memoized row that legitimately
never re-renders still stays current.

Colour. Full status tier regenerated at 90% of gamut max (was 85), same rule as
the muted tier, lightness and hue untouched. File icons are pulled toward neutral
in OKLab behind `getFileIconSvg` — material-icon-theme picks its palette for a
tree where the icon is the only colour on the row, and ours is not. New/deleted
files use the forge square-plus/square-minus glyphs at muted weight instead of
tinted pills. Project icons round a little more; uploaded icons never do.

The project status badge is a knockout again: it fills with the colour of the row
behind it, so the ring reads as a gap in the icon rather than a white halo.

File rows. The always-visible kebab is gone from the file explorer and diff tree
in favour of right-click, which also reclaims the reserved action slot.

docs/floating-panels.md

@@ -258,3 +258,6 @@ Before you write a new one, ask:
    bounded**. Verify before you commit.
 
 Then copy the closest canonical file and trim.
+
+Building a **menu** rather than a bare panel? Don't. Use the menu engine — it already solves
+everything above, plus submenus, sheets, and hover intent. See [menus.md](menus.md).