feat(workbench): Edit single files without opening a project (#145)

Files can now be opened without a project. A file dropped on the Welcome screen, picked with File › Open File…, or passed on the command line or to a second instance opens in a lightweight window: editor tabs only, with no project tree, indexing, language servers, VCS or project tool windows. Syntax highlighting, find, save, undo and Local History still work. More files opened later join the same window as new tabs. Open as Project in the status bar opens the file's directory as a full project, following the open-project policy, and reopens the file there.

Backend: a light session reuses Project with an unindexed index, no LSP or VCS, and a flat watch on each opened file's directory. It is never reused as a project for the same root. Previously a file passed at launch opened its parent directory as a project.

src/workbench/Workbench.module.css

@@ -17,6 +17,28 @@
     var(--bg-frame);
 }
 
+/* LightEdit: the editor as the only island, inset like the project frame's. */
+.lightEditFrame {
+  background: var(--bg-frame);
+}
+.lightEdit {
+  flex: 1;
+  display: flex;
+  min-width: 0;
+  min-height: 0;
+  padding: 0 var(--island-gap);
+}
+.lightEditIsland {
+  flex: 1;
+  display: flex;
+  flex-direction: column;
+  min-width: 0;
+  min-height: 0;
+  overflow: hidden;
+  background: var(--bg-island);
+  border-radius: var(--island-radius);
+}
+
 /* Settings › Appearance › "Use project colors in main toolbar" off. */
 :global(:root[data-project-colors="off"]) .framed {
   background: var(--bg-frame);