Polish the hand-drawn desktop icon glyphs: rounded corners, gradient, blue

Requested directly: default icons "look very rudimentary... make slightly
blue as well, polished look".

Two changes to decoration::render_desktop_icon and its five draw_*_glyph
helpers:

- New fill_rounded_rect primitive: softly rounded corners (the same
  clamp-then-distance smoothstep construction rounded_corners_pixman::
  apply_corner_mask already established for content masking, not a new
  technique) plus a vertical top-to-bottom gradient instead of one flat
  fill - the same light-source cue buttons.rs's own glossy_shade uses
  for the titlebar dots, as a plain linear gradient here. Applied to each
  glyph's main body shape; small details (folder tab edge, computer
  stand, trash ridges, home roof) stay flat/sharp.
- A dedicated ICON_COLOR constant (a clean mid-blue) instead of reading
  theme.titlebar_fg_focused - that field is whatever the user's own
  titlebar accent happens to be configured to, which could be any
  colour; these glyphs want a consistent, recognisable blue palette of
  their own, independent of theme.

Build/test/clippy already verified clean as part of the layer-shell scale
fix commit just before this one (same source tree, installed together).

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