feat: allow selecting multiple folders via Add Folder button

[javi11]

Summary

Fixes #207 — users can now select and upload multiple folders at once using the "Add Folder" button in the desktop (Wails) app.

• Go backend (internal/backend/upload.go): Added SelectFolders() (opens the native multi-select file picker and returns only directory paths) and UploadFolders([]string) (queues each folder as a separate NZB, emitting a single queue-updated event at the end).
• Wails JS bindings (App.js / App.d.ts): Manually added bindings for both new methods per CLAUDE.md rules.
• Unified API client (client.ts): Added selectFolders() and uploadFolders() delegating to the new Wails methods in desktop mode.
• DashboardHeader.svelte: handleFolderUpload now calls selectFolders() + uploadFolders(). Shows a singular toast for 1 folder and a count-aware toast for 2+.
• i18n (en/es/fr/tr): Added folders_added and folders_added_description keys; updated tooltip text to mention multiple folders.

The web-mode code path is unchanged — browsers only expose one folder at a time via webkitdirectory and that remains as-is.

Test Plan

• Click "Add Folder" in the desktop app and select two or more folders (Cmd-click on macOS, Ctrl-click on Windows/Linux) — both should appear as separate items in the upload queue and the toast should say "N folders have been added to the upload queue"
• Select a single folder — toast should still say "Folder has been added to the upload queue"
• Cancel the dialog without selecting anything — no error, no queue change
• go build ./... passes
• bun run check in frontend/ passes (0 errors, 0 warnings)