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/builtin/claude/src/image_thumbnail.dart

@@ -36,7 +36,7 @@ Widget _placeholder(BuildContext context, double size) {
     height: size,
     color: t.panelBackground,
     alignment: Alignment.center,
-    child: ClideIcon(PhosphorIcons.image, size: size * 0.45, color: t.globalTextMuted),
+    child: ClideIcon(PhosphorIcons.byName('image'), size: size * 0.45, color: t.globalTextMuted),
   );
 }