fix: prevent terminal container from overflowing page on desktop

The desktop terminal page sometimes grew an outer scrollbar because the
terminal instance/wrapper/container chain lacked height constraints.
Without min/max-height enforcement, xterm.js's internal viewport could
expand its parent flex/grid track past the available space.

- Add overflow: hidden to .terminal-page.
- Add max-height: 100% and overflow: hidden to .terminal-instance.
- Add max-height: 100% to .terminal-wrapper.
- Add min-height: 0 to .terminal-container.
- Constrain .xterm-viewport to max-height/width 100% so it fills but
  never exceeds its container.

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

Refs: openspec/changes/fix-terminal-container-overflow

apps/web/src/styles/utilities.css

@@ -439,6 +439,7 @@ a.nav-item,
 	min-height: 0;
 	padding: var(--space-4);
 	gap: var(--space-4);
+	overflow: hidden;
 }
 
 .terminal-page-header {
@@ -457,6 +458,7 @@ a.nav-item,
 	grid-template-rows: auto 1fr;
 	flex: 1;
 	min-height: 0;
+	max-height: 100%;
 	border: 1px solid var(--border);
 	border-radius: 10px;
 	overflow: hidden;
@@ -549,6 +551,7 @@ a.nav-item,
 	position: relative;
 	width: 100%;
 	height: 100%;
+	min-height: 0;
 	padding: 0;
 	overflow: hidden;
 }
@@ -557,6 +560,8 @@ a.nav-item,
 .terminal-container .xterm-viewport {
 	touch-action: auto;
 	overscroll-behavior: auto;
+	max-height: 100% !important;
+	width: 100% !important;
 }
 
 /* xterm.js manages its own positioning and sizing */
@@ -767,7 +772,9 @@ a.nav-item,
 .terminal-instance {
 	flex: 1;
 	min-height: 0;
+	max-height: 100%;
 	display: none;
+	overflow: hidden;
 }
 
 .terminal-instance.active {