feat(editor): 文件树「在应用中打开」子菜单（外部编辑器/图钉固定/SSH 远端/自定义编辑器） (#334)

* feat(editor): file tree 'open with' submenu (external editors, pin, SSH-remote, custom editors)

The file tree right-click menu gains an 'Open with >' submenu with the
resource manager (reveal in file manager), VS Code, Cursor and Zed, plus
user-defined custom editors (name + URL template + VSCode-family flag).
Each submenu row carries a pushpin that promotes the target to a direct
top-level menu item.

A configurable SSH host (settings: Files card gear) switches VSCode-family
openers to <scheme>://vscode-remote/ssh-remote+<host>/<path> and hides
host-local targets (file manager / Zed / non-VSCode custom editors).

External opens run through the new fenced /sidebar/api/open.external route
which spawns the platform opener with an argv array (no shell); the client
never navigates raw scheme URLs itself. The settings seam's render panel now
appends after the declarative rows instead of replacing them.

* feat(editor): polish open-with menu UI — brand icons, pushpin, chevron, row alignment

- Submenu rows: the custom label is now a block-level flex row filling the
  primitives itemLabel, so the pushpin docks at the row's far right (matching
  the trailing check of every other menu); menu row icons normalized to 16px
  (DSH native menus use 16, ours were 14).
- The submenu parent row gains a right-aligned chevron (the primitives Menu
  renders no arrow for submenu parents).
- Icons via react-icons: codicons VscPin/VscPinned pushpins (replacing the
  hand-drawn pin), VscFile/VscFolder/VscFolderOpened tree glyphs (replacing
  the </> file mark), VscLinkExternal for the open-with parent and the
  browser tab's open-external button (globally), SiCursor/SiZedindustries
  brand silhouettes; the VS Code mark is inlined from simple-icons@11.0.0
  (CC0) as IconVscode16 since later simple-icons releases dropped it. All
  monochrome currentColor to follow the active skin.
- tsdown: pin react-icons si/vsc to their ESM entries via resolve.alias —
  the package's exports map lists require before import, so the shared
  conditionNames picked the unshakeable CJS entry and bloated the core
  bundle by ~6.4 MB; tree-shaken it now adds only ~12 KB.

src/client/sidebar.module.css

@@ -2521,3 +2521,54 @@ body[data-dsh-sidebar-dragging] .bottomPanel {
     max-width: 128px;
   }
 }
+
+/* The file tree "open with" menu rows: the label area is a BLOCK-level flex
+   row that fills the primitives `.itemLabel` span (which is itself `flex: 1`
+   inside the row), so the trailing control — the pushpin on submenu rows,
+   the chevron on the submenu parent — docks at the row's far right, exactly
+   like the trailing check on selected rows of every other menu. */
+.openWithLabel {
+  display: flex;
+  align-items: center;
+  gap: 8px;
+  width: 100%;
+  min-width: 0;
+}
+
+.openWithName {
+  flex: 1 1 auto;
+  min-width: 0;
+  overflow: hidden;
+  text-overflow: ellipsis;
+  white-space: nowrap;
+}
+
+/* The trailing "expands into a submenu" chevron on the open-with parent row
+   (the primitives Menu renders no arrow for submenu parents). */
+.openWithChevron {
+  flex: none;
+  color: var(--dsw-alias-label-tertiary);
+}
+
+/* The pushpin toggle on an "open with" submenu row: brand ink once the
+   target is pinned to the menu's top level. */
+.openWithPin {
+  flex: none;
+  display: inline-flex;
+  align-items: center;
+  justify-content: center;
+  width: 20px;
+  height: 20px;
+  border-radius: 6px;
+  color: var(--dsw-alias-label-tertiary);
+  cursor: pointer;
+}
+
+.openWithPin:hover {
+  background: var(--dsw-alias-interactive-bg-hover);
+  color: var(--dsw-alias-label-primary);
+}
+
+.openWithPinActive {
+  color: var(--dsw-alias-state-business-primary);
+}