feat: add @ file mention support in chat input and unify directory attachment

Support typing "@" in the chat input to trigger a file list popup with
filtering, keyboard navigation, and file/directory attachment. Directories
from the sidebar file tree now attach as resource links instead of injecting
text into the input.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/lib/path-utils.ts

@@ -0,0 +1,9 @@
+export function joinFsPath(basePath: string, relPath: string): string {
+  if (!relPath) return basePath
+  const separator = basePath.includes("\\") ? "\\" : "/"
+  const normalizedRel = relPath.replace(/[\\/]/g, separator)
+  if (basePath.endsWith("/") || basePath.endsWith("\\")) {
+    return `${basePath}${normalizedRel}`
+  }
+  return `${basePath}${separator}${normalizedRel}`
+}