feat(FN-1493): add directory browsing UX to plugin install flow

- Add DirectoryPicker component for browsing filesystem to select plugin source
- Add dist-folder and package-root install-source resolution in backend
- Wire frontend installPlugin to mode-discriminated endpoint (/plugins/install-local)
- Add comprehensive regression tests for browse-driven plugin install workflow
- Update PluginManager UI with directory picker integration and CSS styles

packages/dashboard/app/styles.css

@@ -28362,3 +28362,157 @@ html .column.drag-over * {
     max-height: 300px;
   }
 }
+
+/* ── Plugin Manager ─────────────────────────────────────────── */
+
+.plugin-manager,
+.plugin-manager-detail {
+  display: flex;
+  flex-direction: column;
+  gap: 12px;
+}
+
+.plugin-manager-header {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+}
+
+.plugin-manager-header h3 {
+  margin: 0;
+}
+
+.plugin-manager-actions {
+  display: flex;
+  gap: 8px;
+  align-items: center;
+}
+
+.plugin-install-form {
+  display: flex;
+  flex-direction: column;
+  gap: 8px;
+  padding: 12px;
+  border: 1px solid var(--border);
+  border-radius: 8px;
+  background: var(--bg-secondary, var(--card-bg));
+}
+
+.plugin-install-hint {
+  margin: 0;
+  font-size: 0.82rem;
+  color: var(--text-secondary, var(--text-muted));
+  line-height: 1.45;
+}
+
+.plugin-install-hint code {
+  padding: 1px 5px;
+  border-radius: 4px;
+  background: var(--bg-tertiary, rgba(127, 127, 127, 0.12));
+  font-size: 0.82em;
+}
+
+.plugin-install-actions {
+  display: flex;
+  gap: 8px;
+  justify-content: flex-end;
+}
+
+.plugin-list {
+  display: flex;
+  flex-direction: column;
+  gap: 2px;
+}
+
+.plugin-item {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  padding: 8px 10px;
+  border-radius: 6px;
+  transition: background 0.15s;
+}
+
+.plugin-item:hover {
+  background: var(--bg-secondary, rgba(127, 127, 127, 0.08));
+}
+
+.plugin-info {
+  display: flex;
+  align-items: center;
+  gap: 8px;
+  min-width: 0;
+}
+
+.plugin-name {
+  font-weight: 500;
+  white-space: nowrap;
+  overflow: hidden;
+  text-overflow: ellipsis;
+}
+
+.plugin-version {
+  font-size: 0.82rem;
+}
+
+.plugin-state-badge {
+  font-size: 0.75rem;
+  font-weight: 600;
+  text-transform: uppercase;
+  letter-spacing: 0.03em;
+}
+
+.plugin-actions {
+  display: flex;
+  align-items: center;
+  gap: 6px;
+  flex-shrink: 0;
+}
+
+/* Detail view */
+
+.plugin-manager-detail-header {
+  display: flex;
+  align-items: center;
+  gap: 8px;
+}
+
+.plugin-manager-detail-header h3 {
+  margin: 0;
+  flex: 1;
+}
+
+.plugin-detail-content {
+  display: flex;
+  flex-direction: column;
+  gap: 16px;
+}
+
+.plugin-detail-meta {
+  font-size: 0.9rem;
+}
+
+.plugin-detail-meta p {
+  margin: 4px 0;
+}
+
+.plugin-description {
+  color: var(--text-secondary, var(--text-muted));
+}
+
+.plugin-detail-section h4 {
+  margin: 0 0 8px;
+}
+
+.plugin-settings-form {
+  display: flex;
+  flex-direction: column;
+  gap: 10px;
+}
+
+.plugin-detail-actions {
+  display: flex;
+  gap: 8px;
+  padding-top: 8px;
+  border-top: 1px solid var(--border);
+}