fix: improve mobile terminal alternate-screen detection and scrolling

- Revert touch-action: none on .xterm-viewport so xterm.js can fall back
  to its own viewport scrolling when the custom handler doesn't take over.
- Detect alternate screen via reference equality
  (term.buffer.active === term.buffer.alternate) instead of the
  string, which could report normal buffer incorrectly.
- Lower vertical-scroll activation threshold from 4px to 2px and only
  prevent default once a vertical gesture is recognized.
- In normal buffer use term.scrollLines() so xterm.js handles the buffer
  scroll consistently; in alternate screen continue sending SGR 1006
  mouse-wheel sequences to tmux/vim.

Quality gates: typecheck, lint clean, npm test -- --run 87 passed.

apps/web/src/styles/utilities.css

@@ -551,14 +551,12 @@ a.nav-item,
 	height: 100%;
 	padding: 0;
 	overflow: hidden;
-	touch-action: none;
-	overscroll-behavior: none;
 }
 
-/* Ensure xterm's internal viewport also disables browser gestures on mobile */
+/* Let xterm.js's own viewport handle touch; we only lock the outer page */
 .terminal-container .xterm-viewport {
-	touch-action: none;
-	overscroll-behavior: none;
+	touch-action: auto;
+	overscroll-behavior: auto;
 }
 
 /* xterm.js manages its own positioning and sizing */
@@ -572,8 +570,6 @@ a.nav-item,
 /* Mobile terminal container - no padding to maximize space */
 .terminal-wrapper.mobile .terminal-container {
 	padding: 0;
-	touch-action: none;
-	overscroll-behavior: none;
 }
 
 .terminal-wrapper.mobile {