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/components/chat/file-mention-menu.tsx

@@ -0,0 +1,62 @@
+"use client"
+
+import { useEffect, useRef } from "react"
+import { File, Folder } from "lucide-react"
+import { cn } from "@/lib/utils"
+import type { FlatFileEntry } from "@/hooks/use-file-tree"
+
+interface FileMentionMenuProps {
+  files: FlatFileEntry[]
+  selectedIndex: number
+  onSelect: (entry: FlatFileEntry) => void
+}
+
+export function FileMentionMenu({
+  files,
+  selectedIndex,
+  onSelect,
+}: FileMentionMenuProps) {
+  const listRef = useRef<HTMLDivElement>(null)
+
+  useEffect(() => {
+    const el = listRef.current?.children[selectedIndex] as
+      | HTMLElement
+      | undefined
+    el?.scrollIntoView({ block: "nearest" })
+  }, [selectedIndex])
+
+  if (files.length === 0) return null
+
+  return (
+    <div
+      ref={listRef}
+      className="absolute bottom-full left-0 right-0 z-50 mb-1 max-h-48 overflow-y-auto rounded-xl border border-border bg-popover p-1 shadow-lg"
+    >
+      {files.map((entry, i) => (
+        <button
+          key={entry.relativePath}
+          type="button"
+          className={cn(
+            "flex w-full items-center gap-2 rounded-lg px-3 py-1.5 text-left text-sm",
+            i === selectedIndex
+              ? "bg-accent text-accent-foreground"
+              : "hover:bg-muted"
+          )}
+          onMouseDown={(e) => {
+            e.preventDefault()
+            onSelect(entry)
+          }}
+        >
+          {entry.kind === "dir" ? (
+            <Folder className="h-3.5 w-3.5 shrink-0 text-muted-foreground" />
+          ) : (
+            <File className="h-3.5 w-3.5 shrink-0 text-muted-foreground" />
+          )}
+          <span className="truncate font-mono text-xs">
+            {entry.relativePath}
+          </span>
+        </button>
+      ))}
+    </div>
+  )
+}