Charcoal colour palette and improved terminal resize on connect

Theme:
- Replace all purple/blue-tinted backgrounds (#1a1a2e, #1f1f35, etc.)
  with true neutral charcoal greys (#1c1c1c, #232323, #2a2a2a, etc.)
- Neutralise text colours (remove blue tint from secondary/tertiary)
- Update xterm.js theme and session overlay to match

Terminal resize:
- Add fitSession(sessionId) method for targeted terminal fitting
- Fire fit at 30ms, 150ms, and 500ms after 'connected' state event
- Ensures terminal fills container even when layout settles late

frontend/src/js/app.js

@@ -134,9 +134,13 @@ class App {
         window.runtime.EventsOn('session:state', (payload) => {
             this.tabs.updateSessionState(payload.sessionID, payload.state, payload.error);
             this.updateStatusBar();
-            // Re-fit the terminal once connected so it picks up the final container size
+            // Re-fit the terminal once connected so it picks up the final container size.
+            // Use multiple delays because the panel layout may still be settling.
             if (payload.state === 'connected') {
-                setTimeout(() => this.terminals.fitActive(), 50);
+                const fit = () => this.terminals.fitSession(payload.sessionID);
+                setTimeout(fit, 30);
+                setTimeout(fit, 150);
+                setTimeout(fit, 500);
             }
         });