File ops: Redesign type-mismatch conflict dialog

Rebuild the conflict dialog on top of the original file→file shape (filename, "Existing:" / "New:" rows, the 4×2 button grid, the bottom Rollback row) so every clash type reads the same way. The variants only differ where the spec actually wants them to:

- File→file and folder→folder stay baseline.
- Folder→file: row labels switch to `Existing (file):` and `New (folder):`; no warning, `Overwrite` label unchanged.
- File→folder: row labels switch to `Existing (folder):` and `New (file):`; a red `role="alert"` block sits below the title and above the filename, calling out that the entire target folder would be deleted; `Overwrite` becomes `Overwrite folder with file`, `Overwrite all` becomes `Overwrite folders with files`. When `destinationSize === null`, the Existing-size slot renders `(unknown)` in muted color and `Overwrite all smaller` disables (with a tooltip) since there's no folder size to compare against. `Overwrite all older` always stays enabled — mtime is always available.

Wire-shape: `WriteConflictEvent.destinationSize` and `sizeDifference` shift to `number | null` to match the BE event the index-backed dir-size lookup now emits. The hand-maintained shim in `apps/desktop/src/lib/file-explorer/types.ts` is the only place this needs to land; `bindings.ts` doesn't carry this type.

Drop the agent's earlier "Currently / Incoming" cards + danger-styled "Replace folder" / "Replace file" buttons and their CSS (`.mismatch-cards`, `.mismatch-card*`, `.conflict-lede`) — they forked the layout where the spec wants one consistent shape.

Test coverage: `TransferProgressDialog.conflict.test.ts` walks a `describe.each`-driven matrix over the four clash types plus a known-size / null-size axis for the file→folder case, mounts the dialog component directly, captures the `onWriteConflict` callback from the mock, and asserts row labels, button copy, warning markup, disabled state, and one axe-core run per variant. 32 tests, ≈25 ms each in jsdom.

Author: vdavid

apps/desktop/src/lib/file-explorer/types.ts

@@ -554,18 +554,21 @@ export interface WriteConflictEvent {
   operationId: string
   sourcePath: string
   destinationPath: string
-  /** Source file size in bytes */
+  /** Source file size in bytes. Always known from the pre-flight scan. */
   sourceSize: number
-  /** Destination file size in bytes */
-  destinationSize: number
+  /** Destination size in bytes. `null` when the destination is a folder whose
+   *  size the drive index hasn't crawled yet (network mount, MTP, fresh paths) —
+   *  rendered as `(unknown)` in the UI. */
+  destinationSize: number | null
   /** Source modification time (Unix timestamp in seconds), if available */
   sourceModified: number | null
   /** Destination modification time (Unix timestamp in seconds), if available */
   destinationModified: number | null
   /** Whether destination is newer than source */
   destinationIsNewer: boolean
-  /** Size difference (positive = destination is larger) */
-  sizeDifference: number
+  /** `destinationSize - sourceSize` when both are known. Collapses to `null`
+   *  when `destinationSize` is `null` (no folder size to subtract against). */
+  sizeDifference: number | null
   /** Whether the source side is a directory. Renders the type-mismatch warning when this differs from `destinationIsDirectory`. */
   sourceIsDirectory: boolean
   /** Whether the destination side is a directory. See `sourceIsDirectory`. */