Context/desktop menu polish: real hover tint, real separator line, Select All

Reported live: "looks weird and unpolished... need a lot more items".
Compared directly against the exact AGS reference this project's own
menu rebuild already targets rather than guessing:

- Highlighted rows used a flat, fully-saturated fill instead of the
  reference's subtle 22%-accent-into-background wash. New decoration::
  color::mix_rgb (channel-wise linear blend, generalizing brighten/
  darken's fixed-target blends to an arbitrary second colour/ratio) lets
  render_context_menu reproduce that same ratio.
- Every separator row was a label string of Unicode box-drawing
  characters rendered as text glyphs, which render inconsistently at
  small sizes - a label that's entirely U+2500 now draws a real 1px
  hairline instead; a label that mixes it with real text ("--- Move to
  Workspace ---", a deliberate section-header convention) still renders
  as text, unchanged.
- "Select All" added to the bare-desktop menu, the one action every
  mainstream desktop's own menu offers that this one lacked.

New tests needed real care: the panel's own rounded-corner distance
field softens alpha within its radius of any canvas edge, not just the
visible corners, so a naive full-row pixel scan against bg picked that
up as a false positive on the first attempt - fixed by scanning only
rows/columns confirmed (via a throwaway debug dump) to sit inside the
panel's genuinely flat interior.

Full workspace build/test/clippy clean, built and installed. Real
submenus and per-row icons remain real, separate scope - this
project's floating-menu UI has no nested-panel concept yet.

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