test(files): cover 409 askConflict keep-both path

Tries to create `cfg`, which collides with the seeded directory of the
same name. /files/save returns 409 "destination is not a file";
routedSaveClicked routes that through fo.askConflict, which opens the
inline #files-conflict-modal on top of the still-open routed editor.
Clicking keep-both triggers a second POST with the suffixed path
(`cfg (1)`), the routed modal closes, and the new row materialises in
the tree.

This is the F4 path from 8dc14f0 ("wire askConflict into the routed
new-file 409 path"). Before that commit, the routed code branch fell
through to a generic alert(). With this test in place, a missing
call site fails loudly instead of silently.

Pins three invariants:
  * The conflict dialog is INLINE, not routed — it appears without a
    URL change (the decision tree in AGENTS.md "Modals: inline vs
    routed" hinges on this).
  * .files-conflict-path echoes the original colliding path, not the
    computed suffix — the suffix is internal, the user sees the
    collision.
  * withCollisionSuffix("cfg") → "cfg (1)" (no dot after the last
    slash → trailing-suffix branch in uploads.js).

Per docs/superpowers/plans/2026-05-17-files-overlay-e2e-handoff.md.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

l4d2web/tests/e2e/test_files_overlay.py

@@ -116,3 +116,62 @@ def test_create_new_file_routed(page: Page, files_overlay_server) -> None:
     expect(
         page.locator(f'.file-tree-row-file[data-target-path="{new_filename}"]')
     ).to_be_visible(timeout=5000)
+
+
+def test_create_new_file_409_askConflict_keep_both(page: Page, files_overlay_server) -> None:
+    """Try to create a file named `cfg`, which collides with the
+    seeded directory of the same name. /files/save returns 409 because
+    the target exists and is not a file; routedSaveClicked routes that
+    409 through fo.askConflict, which opens the INLINE
+    #files-conflict-modal on top of the still-open routed editor.
+    Clicking "keep-both" causes a second POST with the suffixed path
+    (cfg → "cfg (1)"), the routed modal closes, and the new row
+    appears in the tree.
+
+    This is the F4 path from commit 8dc14f0 ("wire askConflict into
+    the routed new-file 409 path"). Without coverage it can regress
+    silently — the legacy create-new flow went through a different code
+    path before the rewrite, and a missing call site would only
+    manifest as a confusing alert() instead of the conflict dialog.
+
+    Key invariants this asserts:
+      * The conflict dialog is inline, NOT routed — it appears WITHOUT
+        a URL change (we don't navigate, we just wait for the dialog).
+      * .files-conflict-path shows the original colliding path, not
+        the suffixed one.
+      * withCollisionSuffix("cfg") returns "cfg (1)" (single-extension
+        path with no dot falls into the trailing-suffix branch).
+    """
+    base = files_overlay_server["base_url"]
+    overlay_id = files_overlay_server["overlay_id"]
+    overlay_root = files_overlay_server["overlay_root"]
+    _open_overlay(page, base, overlay_id)
+
+    page.click('button[data-action="new-file"][data-target-path=""]')
+    _wait_for_routed_editor(page)
+
+    page.fill('input[data-editor-filename]', "cfg")
+    new_content = "STEAM_1:0:5\n"
+    page.evaluate("(text) => window.__filesEditor.setContent(text)", new_content)
+    page.click(".files-editor-save")
+
+    # The conflict dialog should pop up because `cfg` is a directory.
+    conflict = page.locator("#files-conflict-modal")
+    expect(conflict).to_be_visible(timeout=5000)
+    # The dialog must echo the ORIGINAL colliding path, not the suffix.
+    expect(conflict.locator(".files-conflict-path")).to_have_text("cfg")
+
+    page.click('[data-files-conflict-action="keep-both"]')
+
+    # Both modals should close (conflict dialog first, routed modal
+    # after the second /files/save resolves).
+    expect(page.locator("#modal-container")).to_be_hidden(timeout=5000)
+    expect(conflict).to_be_hidden(timeout=5000)
+
+    # The seeded `cfg/` directory must still be intact + the new
+    # suffixed file landed alongside it.
+    assert (overlay_root / "cfg").is_dir()
+    assert (overlay_root / "cfg (1)").read_text() == new_content
+    expect(
+        page.locator('.file-tree-row-file[data-target-path="cfg (1)"]')
+    ).to_be_visible(timeout=5000)