style(ui): enforce square corners and preserve circular markers

Introduce a square UI language by resetting border-radius on
controls and surfaces to create consistent square corners app-wide.
Add explicit exceptions to preserve circular markers and keep
branch-flag shapes so avatars, dots, and graph connections remain
recognizable. Remove a few unnecessary !important modifiers to
simplify overrides.

- Reset border-radius globally for elements and pseudo-elements.
- Preserve circular avatars, graph dots, and similar markers.
- Keep curved branch flag shapes and targeted corner rules.

src/app.css

@@ -2824,7 +2824,7 @@
     padding: 0 5px;
     border-color: color-mix(in srgb, var(--ref-lane-color, #69a7ff) 34%, transparent);
     border-left-width: 2px;
-    border-radius: 0 5px 5px 0 !important;
+    border-radius: 0 5px 5px 0;
     color: #dce9ff;
     background:
       linear-gradient(90deg, color-mix(in srgb, var(--ref-lane-color, #69a7ff) 14%, rgba(18,24,36,.96)), rgba(18,24,36,.82));
@@ -6285,7 +6285,7 @@ input:focus, textarea:focus, select:focus { box-shadow: 0 0 0 3px color-mix(in s
   contain-intrinsic-block-size: 108px;
 }
 .commit-avatar { border-radius: 50%; }
-.commit-kind, .compact-ref-chip { border-radius: 4px !important; }
+.commit-kind, .compact-ref-chip { border-radius: 4px; }
 .workspace-statusbar {
   display: flex;
   align-items: center;
@@ -7366,6 +7366,45 @@ input:focus, textarea:focus, select:focus { box-shadow: 0 0 0 3px color-mix(in s
   background: var(--color-border);
 }
 .ai-review-suggestion { display: grid; gap: 3px; margin-top: 9px; padding: 8px 9px; border-radius: 5px; background: var(--color-surface-dim); }
+
+/* Square UI language: controls and surfaces have no rounded corners. */
+*,
+*::before,
+*::after {
+  border-radius: 0 !important;
+}
+
+/* Preserve shapes that communicate a circular marker rather than a rounded surface. */
+.commit-avatar,
+.repo-switch > span,
+.commit-ref-detail-item > i,
+.graph-dot,
+.worktree-rail span,
+.workspace-health > span,
+.workspace-auto i {
+  border-radius: 50% !important;
+}
+
+/* Branch flags keep their flag shape and curved graph connection. */
+.compact-ref-chip,
+.compact-ref-overflow {
+  border-radius: 5px !important;
+}
+.compact-ref-chip.branch {
+  border-radius: 0 5px 5px 0 !important;
+}
+.branch-ref-cluster.local-only .compact-ref-chip.branch {
+  border-radius: 0 !important;
+}
+.compact-ref-local-marker {
+  border-radius: 0 5px 5px 0 !important;
+}
+.commit-body.has-branch-ref::before {
+  border-top-left-radius: 14px !important;
+}
+.commit-body.has-branch-ref::after {
+  border-bottom-right-radius: 14px !important;
+}
 .ai-review-suggestion strong { color: var(--color-ink-dim); font-size: 9.5px; text-transform: uppercase; letter-spacing: .05em; }
 .ai-review-suggestion span { color: var(--color-ink-muted); font-size: 11.5px; line-height: 1.45; }
 .ai-review-clean-state { display: grid; place-items: center; align-content: center; min-height: 280px; gap: 8px; color: #2da44e; text-align: center; }