docs(js-sdk): document resumeUpload, recoveryToken, and FileState

[dobby-coder]

Closes #83.

Adds a Resume an interrupted upload section to docs/sdk/js-encryption.md covering the new public exports from encryption4all/postguard-js#66:

• FileState shape and the two persistable fields (uuid, recoveryToken).
• resumeUpload(cryptifyUrl, uuid, recoveryToken, signal?) signature, return type, and wire-field mapping (cryptify_token → state.token, prev_token → state.prevToken, X-Recovery-Token request header).
• 404 with upload_session_not_found surfaces as UploadSessionExpiredError; cryptify collapses unknown UUID and wrong recovery token into the same response.

Amends docs/sdk/js-errors.md so the "no resume path" line scopes to the post-expiry case and cross-links the new section.

Source links pinned to postguard-js merge commit 6205fc309a.

Honest gap

resumeUpload and FileState are re-exported by @e4a/pg-js, but pg.encrypt(...).upload() does not yet surface recoveryToken on UploadResult. Capturing the token from the public API needs a follow-up on postguard-js, tracked at encryption4all/postguard-js#68. The new docs section calls this out explicitly so readers are not misled into thinking the high-level path is wired up.

Verification

• npm install && npm run docs:build succeeds in ~6s.
• Source link URL HTTP 200 at the pinned commit (curl -sI).
• No banned words, em dashes, or banned transitions introduced (per postguard-docs/CLAUDE.md style rules).

Reviewer quickstart

```
git fetch origin && git checkout docs/js-sdk-resume-upload && npm install && npm run docs:dev
```

[dobby-coder]

LGTM (cannot self-approve). Verified against postguard-js@6205fc309a:

• FileState shape in the table (token / prevToken / uuid / recoveryToken) matches src/api/cryptify.ts#L13-L33, including the prevToken-undefined-until-first-commit detail.
• resumeUpload signature, return type, X-Recovery-Token header, and wire-field mapping (cryptify_token → state.token, optional prev_token → state.prevToken) match L143-L178.
• Both symbols are re-exported from postguard-js src/index.ts.
• UploadSessionExpiredError anchor target exists in docs/sdk/js-errors.md#L51.
• postguard-js#68 is open and tracks the UploadResult.recoveryToken follow-up — the 'Current limitation' callout is accurate, not stale.
• npm run docs:build succeeds (~7s) on this branch.
• No banned words, em dashes, or banned transitions introduced in the diff.

The errors-page edit correctly narrows the 'no resume path' line to the post-expiry case and cross-links the new section.