Convert chrome-local color constants to theme-aware accessors

The previous theme refactor only flipped `colors::*` tokens; the
~50 chrome-local `const FOO: u32 = 0xffffff…;` overlays kept their
white-glass values, so dark mode still painted a light omnibar pill,
light search field, light sidebar tab card, light command-overlay
backdrop, light sync page chips, etc.

Promote `colors::pick(light, dark)` to `pub` so component-internal
constants can reach the same thread-local mode the global palette
uses, then convert every white-glass (`0xffffff??`) and warm-dark
wash (`0x281e14??`) constant in:

- chrome/topbar (omnibar pill, hover chip)
- chrome/sidebar_chrome + chrome/sidebar (rail nav row hover/active,
  highlight border, close-button hover, unread badge)
- chrome/sidebar_header (workspace picker, disclosure card)
- chrome/command_overlay + chrome/command_rows + chrome/command_footer
- chrome/settings_layout, chrome/appearance_form,
  chrome/appearance_layout_cards, chrome/plugin_detail_view
- chrome/home/style (search field, arrow chip, pill, card, add-tile)
- internal_pages/sync (status pill, metrics card, button row)
- internal_pages/plugin_catalog
- shell/render (main pane highlight border)

Each constant becomes `fn name() -> u32 { colors::pick(<light>, <dark>) }`
where the dark counterpart preserves the original alpha but swaps the
warm-white prefix `ffffff` for warm-graphite `1f1d1b` (or the inverse:
`281e14` warm-dark wash → `f2efe9` warm-white wash). Brand accents
(`0xc96442??`, `0xffaa7733`) and the global slate backdrop keep their
single value across modes.

crates/ely_app/src/shell/render.rs

@@ -192,7 +192,7 @@ impl ElyShell {
             .rounded(px(spacing::RADIUS_CARD))
             .bg(rgba(panel_color))
             .border_1()
-            .border_color(rgba(MAIN_PANE_HIGHLIGHT_BORDER))
+            .border_color(rgba(main_pane_highlight_border()))
             .shadow(panel_shadow())
             .overflow_hidden()
             .child(render_topbar_chrome(self, snapshot, active_tab, sidebar_collapsed, window, cx))
@@ -339,7 +339,9 @@ impl ElyShell {
 /// substitute for the design's `box-shadow: inset 0 0 0 1px rgba(255,255,255,0.5)`.
 /// Painted as the panel's own border so it stays part of the frame and
 /// never participates in hit testing.
-const MAIN_PANE_HIGHLIGHT_BORDER: u32 = 0xffffff80;
+fn main_pane_highlight_border() -> u32 {
+    colors::pick(0xffffff80, 0x1f1d1b80)
+}
 
 /// Lower clamp for the live sidebar-resize drag. Stays a touch above
 /// the per-space DEFAULT so a casual flick doesn't snap the user to a