chore: stop treating dirs as having size 128 (#10750)

Resp. directories for the database, and both dirs and symlinks for
`FileInfo.FileSize`. Instead handle it in the UI progress percentage. We
even already have special cases there for deletions, might as well
handle directories just like any other zero-sized needed item there.

This went through the very thorough testing of running it on my laptop,
the migration was applied and it seemed to be working fine after.

---------

Signed-off-by: Simon Frei <freisim93@gmail.com>

internal/db/sqlite/sql/migrations/folder/06-zero-size-dirs.sql

@@ -0,0 +1,12 @@
+-- Copyright (C) 2026 The Syncthing Authors.
+--
+-- This Source Code Form is subject to the terms of the Mozilla Public
+-- License, v. 2.0. If a copy of the MPL was not distributed with this file,
+-- You can obtain one at https://mozilla.org/MPL/2.0/.
+
+-- All non-file entries should have size zero.
+-- Directories were previously stored with a "synthetic" size of 128.
+UPDATE files
+    SET size = 0
+    WHERE type != 0
+;