fix(git): make unsetting upstream idempotent

Saving sync settings with no upstream could cause a fatal Git
error when unsetting upstream on a branch that never had tracking
information. This change guards the operation by checking for an existing
merge configuration before unsetting, making it idempotent. A test was
added to verify that clearing an unconfigured upstream is a no-op.

- Add test ensuring clearing an unconfigured upstream is a no-op

src/app.css

@@ -8672,3 +8672,169 @@ input:focus, textarea:focus, select:focus { box-shadow: 0 0 0 3px color-mix(in s
   .branch-filter-summary { align-items: stretch; flex-direction: column; }
   .branch-filter-actions { display: grid; grid-template-columns: repeat(3, minmax(0, 1fr)); }
 }
+
+/* Authentication and token dialogs ------------------------------------
+   Keep credentials, access tokens and API-key forms on the active theme
+   instead of the legacy violet/black palette. */
+.cred-card,
+:root[data-theme="light"] .cred-card {
+  border-color: var(--color-border-input);
+  background: var(--app-dialog-bg);
+  box-shadow: var(--app-dialog-shadow);
+}
+
+.cred-hero,
+:root[data-theme="light"] .cred-hero {
+  border-bottom-color: var(--color-border);
+  background:
+    radial-gradient(circle at 8% 0%, color-mix(in srgb, var(--color-accent) 14%, transparent), transparent 44%),
+    var(--app-dialog-chrome);
+}
+
+.cred-hero-icon,
+:root[data-theme="light"] .cred-hero-icon {
+  border-color: color-mix(in srgb, var(--color-accent) 42%, var(--color-border));
+  color: var(--color-accent);
+  background: color-mix(in srgb, var(--color-accent) 12%, var(--color-surface-raised));
+  box-shadow: 0 10px 24px color-mix(in srgb, var(--color-accent) 18%, transparent);
+}
+
+.cred-hero-label,
+:root[data-theme="light"] .cred-hero-label {
+  color: var(--color-accent);
+}
+
+.cred-hero-title,
+:root[data-theme="light"] .cred-hero-title {
+  color: var(--color-ink);
+}
+
+.cred-hero-copy,
+:root[data-theme="light"] .cred-hero-copy {
+  color: var(--color-ink-muted);
+}
+
+.cred-security-note,
+:root[data-theme="light"] .cred-security-note {
+  border-color: color-mix(in srgb, var(--color-accent) 32%, var(--color-border));
+  color: var(--color-ink-muted);
+  background: color-mix(in srgb, var(--color-accent) 8%, var(--color-surface-raised));
+}
+
+.cred-security-note svg,
+:root[data-theme="light"] .cred-security-note svg {
+  color: var(--color-accent);
+}
+
+.cred-close,
+:root[data-theme="light"] .cred-close {
+  border-color: var(--color-border);
+  color: var(--color-ink-faint);
+  background: var(--color-surface-raised);
+}
+
+.cred-close:hover:not(:disabled),
+:root[data-theme="light"] .cred-close:hover:not(:disabled) {
+  border-color: var(--color-border-input);
+  color: var(--color-ink);
+  background: var(--color-surface-hover);
+}
+
+.cred-body,
+:root[data-theme="light"] .cred-body {
+  background: var(--app-dialog-bg);
+}
+
+.cred-segment,
+:root[data-theme="light"] .cred-segment {
+  border-color: var(--color-border);
+  background: var(--color-surface-alt);
+}
+
+.cred-seg-btn,
+:root[data-theme="light"] .cred-seg-btn {
+  color: var(--color-ink-faint);
+}
+
+.cred-seg-btn:hover:not(.active),
+:root[data-theme="light"] .cred-seg-btn:hover:not(.active) {
+  color: var(--color-ink);
+  background: var(--color-surface-hover);
+}
+
+.cred-seg-btn.active,
+:root[data-theme="light"] .cred-seg-btn.active {
+  border-color: color-mix(in srgb, var(--color-accent) 46%, var(--color-border));
+  color: var(--color-ink);
+  background: color-mix(in srgb, var(--color-accent) 12%, var(--color-surface-raised));
+  box-shadow: inset 0 -2px 0 var(--color-accent);
+}
+
+.cred-input input,
+.cred-expiry input[type="date"],
+:root[data-theme="light"] .cred-input input,
+:root[data-theme="light"] .cred-expiry input[type="date"] {
+  border-color: var(--color-border-input);
+  color: var(--color-ink);
+  background: var(--app-input-bg);
+  color-scheme: var(--app-color-scheme);
+}
+
+.cred-input input::placeholder,
+:root[data-theme="light"] .cred-input input::placeholder {
+  color: var(--color-ink-faint);
+  opacity: 0.72;
+}
+
+.cred-token-hint,
+:root[data-theme="light"] .cred-token-hint {
+  border-color: color-mix(in srgb, var(--color-accent) 34%, var(--color-border));
+  color: var(--color-ink-muted);
+  background: color-mix(in srgb, var(--color-accent) 8%, var(--color-surface-dim));
+}
+
+.cred-token-hint code,
+:root[data-theme="light"] .cred-token-hint code {
+  color: var(--color-accent);
+  background: color-mix(in srgb, var(--color-accent) 12%, var(--color-surface-raised));
+}
+
+.cred-footer {
+  border-top-color: var(--color-border-subtle);
+}
+
+.cred-save input[type="checkbox"] {
+  accent-color: var(--color-accent);
+}
+
+.cred-save span {
+  color: var(--color-ink-muted);
+}
+
+.cred-cancel {
+  border-color: var(--color-border);
+  color: var(--color-ink-muted);
+  background: var(--color-surface-raised);
+}
+
+.cred-submit {
+  border-color: color-mix(in srgb, var(--color-accent) 62%, var(--color-border));
+  color: var(--color-surface-solid);
+  background: var(--color-primary-dark);
+  box-shadow: none;
+}
+
+.cred-submit:hover:not(:disabled) {
+  border-color: var(--color-accent);
+  color: var(--color-surface-solid);
+  background: var(--color-primary);
+}
+
+@media (max-width: 560px) {
+  .cred-card { width: calc(100vw - 16px); max-height: calc(100vh - 16px); }
+  .cred-hero { padding: 16px; }
+  .cred-body { padding: 16px; }
+  .cred-security-note { align-items: flex-start; border-radius: 8px; padding-block: 7px; }
+  .cred-footer { align-items: stretch; flex-direction: column; }
+  .cred-btns { display: grid; grid-template-columns: 1fr 1fr; }
+}