[device25]

Asked by the owner after #34: open a folder with the tree and its photos, instead of building a zip.

• «Открыть папку». A button next to «Открыть файл», on the start screen, in the header and in the settings sheet. It uses webkitdirectory. It is not shown on phones, which cannot pick a folder.
• Dropping a folder on the window walks it with webkitGetAsEntry. Each file keeps its path inside the folder as relativePath, which the loader reads like webkitRelativePath. Hidden folders (.git, .claude) and __MACOSX are not walked, so a project folder with thousands of git files drops quickly.
• Reading. As in perf(load): open large archives without reading them into memory #34, the tree is the .ged nearest the top, outside hidden folders. Photos are shown straight from disk; nothing is read until shown.
• Wording. The start screen and the drop hint now mention folders.

Tests:

• unit src/ui/folders.test.ts: walking, paths, hidden folders skipped, plain files unchanged;
• e2e tests/e2e/open-folder.spec.ts with the fixture tests/fixtures/synthetic/folder-tree, which has a hidden copy of the tree;
• the drop test's text is updated;
• macOS snapshots updated; Linux snapshots will come from CI.

npm run check:all passes locally.

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