add Phosphor Icons font and wire into icon rails

Phosphor Icons v2.0.8 (MIT) — regular, bold, and fill weights
bundled under assets/fonts/phosphor/. PhosphorIconPainter bridges
the font glyphs into the existing ClideIconPainter interface via
Paragraph API so icon rails, pane headers, and any widget using
ClideIcon can consume them. Sidebar and context panel icon rails
now use Phosphor glyphs instead of hand-painted CustomPaint icons.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/pubspec.yaml

@@ -70,6 +70,7 @@ flutter:
     - assets/LICENSE
     - assets/fonts/jetbrains_mono/OFL.txt
     - assets/fonts/josefin_sans/OFL.txt
+    - assets/fonts/phosphor/LICENSE
     - assets/logo/clide-logo.svg
     - assets/logo/clide-logo-16.png
     - assets/logo/clide-logo-32.png
@@ -97,6 +98,13 @@ flutter:
         - asset: assets/fonts/josefin_sans/JosefinSans-Italic-VariableFont.ttf
           style: italic
 
+    - family: Phosphor
+      fonts:
+        - asset: assets/fonts/phosphor/Phosphor.ttf
+        - asset: assets/fonts/phosphor/Phosphor-Bold.ttf
+          weight: 700
+        - asset: assets/fonts/phosphor/Phosphor-Fill.ttf
+
     - family: JetBrainsMono
       fonts:
         - asset: assets/fonts/jetbrains_mono/JetBrainsMono-Regular.ttf