fix: URL-encode filenames with special characters to prevent 404 errors

[camathieu]

What

Properly encodes filenames with encodeURIComponent (JS) and url.PathEscape (Go) before interpolating them into URL paths across the webapp, Go SDK, and server responses.

Why

Previously, filenames containing characters like #, spaces, or parentheses were interpolated verbatim into URL paths. A filename like #test.txt would send a request to /file/ID/ID/, treating #test.txt as a URL fragment delimiter, resulting in a 404 Not Found error.

Changes

• Webapp (api.js): Added encodeURIComponent() for uploadFile, removeFile, getFileURL, and getArchiveURL.
• Go SDK (plik/internal.go, plik/file.go): Added url.PathEscape() for all upload, download, and remove URL constructions.
• Server (add_file.go): Added url.PathEscape() to the quick-mode URL response. Renamed local url string variable to fileURL to prevent shadowing the net/url import.

Testing

• Added TestUploadFileWithSpecialChars and TestQuickUploadWithSpecialChars in plik/z1_e2e_test.go to verify URL-encoding end-to-end.
• Added uploads a file with special characters in name in webapp/e2e/upload.spec.js to verify the web UI upload flow.
• Ran and passed full regression test suite (make test and make test-frontend-e2e).

[bodji]

LGTM, nice :)