feat: add custom HeaderPanel widget and switchable agent modes

Replace built-in Header with a custom HeaderPanel showing model name,
mode badge, and live token usage. Add AgentMode enum (normal/plan/auto)
with mode-aware permission gating — Plan mode restricts to read-only
tools, Auto mode auto-approves everything. Includes /mode slash command
and Ctrl+P keybinding to cycle modes.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/ui/styles.tcss

@@ -55,8 +55,8 @@ Screen {
 Input {
     dock: bottom;
     margin: 0;
+    border: none;
+    padding: 0 1;
 }
 
-Header {
-    dock: top;
-}
+/* HeaderPanel styles are in DEFAULT_CSS on the widget itself */