feat(web): Open HTML in browsers with a built-in server; add Emmet

- ide-web: built-in static server on 127.0.0.1 that serves project files,
  plus detection and launching of installed browsers.
- Open In › Browser in the editor, project view and tab menus; browser bar
  and built-in preview beside HTML editors; ⌃⇧R opens the default browser.
- Emmet: Tab expansion in markup editors and Update Tag with Emmet.

Claude-Session: https://claude.ai/code/session_01SaPPsVPuk6ZTNsoUCNCbMv

src/features/web/Web.module.css

@@ -0,0 +1,52 @@
+.split {
+  flex: 1;
+  min-height: 0;
+  min-width: 0;
+  display: flex;
+}
+.editor {
+  flex: 1 1 50%;
+  min-width: 0;
+  display: flex;
+  flex-direction: column;
+  position: relative;
+}
+.withPreview .editor {
+  border-right: 1px solid var(--border-editor-tabs, var(--border));
+}
+
+/* Below the inspection widget; shown while the pointer is over the editor, like IDEA. */
+.bar {
+  position: absolute;
+  top: 32px;
+  right: 20px;
+  z-index: 5;
+  display: flex;
+  gap: 1px;
+  padding: 2px;
+  border: 1px solid var(--border-popup);
+  border-radius: var(--radius-toolbar-button);
+  background: var(--bg-popup);
+  box-shadow: var(--shadow-popup);
+  opacity: 0;
+  transition: opacity 120ms ease;
+}
+.editor:hover .bar {
+  opacity: 0.85;
+}
+.bar:hover,
+.bar:focus-within {
+  opacity: 1;
+}
+
+.preview {
+  flex: 1 1 50%;
+  min-width: 0;
+  border: 0;
+  background: var(--bg-editor, var(--bg-app));
+}
+.message {
+  padding: 16px;
+  color: var(--fg-muted);
+  font-family: var(--font-ui);
+}