This commit fixes two bugs that prevent file uploads to Proton Drive:

--- Bug 1: handleRevisionConflict ignores ReplaceExistingDraft when
    GetRevisions returns 2501 ---

When a previous upload attempt fails after the file draft is created but
before blocks are committed, a broken draft remains. On the next attempt
the draft is found (2500 "file already exists"), and GetRevisions is called
to decide what to do with it. However, broken drafts return 422/2501 from
the /revisions endpoint, causing handleRevisionConflict to return an error
immediately — even when ReplaceExistingDraft=true.

Fix: if GetRevisions fails AND ReplaceExistingDraft is set AND the link is
in draft state, treat it as a broken draft and delete the link so the caller
can retry from scratch.

--- Bug 2: block uploads missing required Verifier.Token ---

Proton's storage backend now requires a Verifier.Token per block in the
POST /drive/blocks request. Without it, the storage server rejects block
uploads with HTTP 422 / Code=200501 "Operation failed: Please retry".

The token is computed by fetching a VerificationCode for the revision via:
  GET /drive/v2/volumes/{volumeID}/links/{linkID}/revisions/{revisionID}/verification
then XOR-ing it with the leading bytes of each block's ciphertext (algorithm
sourced from the official Proton Drive JS SDK in ProtonDriveApps/sdk).

Also bumps go-proton-api to v1.0.1-0.20260218123427-1a63a293e3a2 which
updates the default API host from mail.proton.me/api to drive-api.proton.me.

Note: the JS SDK performs an additional client-side decryption check before
computing the XOR token (to detect bit-flips / bad hardware). That step is
not implemented here; the server-side manifest signature still provides
end-to-end integrity verification. A future improvement could add it.

This fix was identified and generated with Claude Code (AI assistant) by a
non-programmer user (GitHub: lmwashere). It has not been independently
reviewed by a Go or cryptography expert. Expert review before merging is
strongly recommended.

Reproducer: rclone copy <file> proton: --verbose
Expected:   upload succeeds
Actual:     422 POST fra-storage.proton.me/storage/blocks (Code=200501)
            followed by 422 GET .../revisions (Code=2501) on retry