Phosphor icons: resolve by name via a generated map (T-314)

Replace the 49 hand-maintained named consts with one generated
label→codepoint map (phosphor_glyphs.g.dart, 1512 glyphs from the glyph
table via tool/gen_phosphor_glyphs.dart). Feature code now references
glyphs by their exact kebab-case name — PhosphorIcons.byName('folder') —
with no raw codepoints; this also lets a Lua extension name an icon
without crossing the FFI boundary with a codepoint.

byName is total: an unknown name degrades to the `placeholder` box so the
bug is visible (it's a real error), while phosphor_glyphs_test asserts
every byName('...') literal in lib/ resolves — recovering the typo check a
const gave. Migrated the 89 call sites. Adds EmptyIconPainter for an
intentional blank that still reserves the icon box; ClideFilterBox gains
showIcon to keep the slot aligned when blank.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/widgets/src/clide_icon.dart

@@ -11,6 +11,23 @@ abstract class ClideIconPainter {
   void paint(Canvas canvas, Color color);
 }
 
+/// Draws nothing, but [ClideIcon] still reserves its `size×size` box — so a
+/// caller can keep the icon slot for alignment without showing a glyph. This is
+/// the *intentional* empty (e.g. a non-search filter field), distinct from an
+/// unknown glyph name, which renders a visible `placeholder` error box (T-314).
+class EmptyIconPainter extends ClideIconPainter {
+  const EmptyIconPainter();
+
+  @override
+  void paint(Canvas canvas, Color color) {}
+
+  @override
+  bool operator ==(Object other) => other is EmptyIconPainter;
+
+  @override
+  int get hashCode => (EmptyIconPainter).hashCode;
+}
+
 class ClideIcon extends StatelessWidget {
   const ClideIcon(
     this.painter, {