Add append semantics via PATCH /post for non-destructive pad updates

[Copilot]

This plugin currently only exposes replace semantics (setText) through POST /post. This change adds append semantics so callers can add content without overwriting existing pad text, while preserving create-on-missing behavior.

• API behavior

  • Added PATCH /post route mapped to append behavior.
  • Kept existing POST /post behavior unchanged (replace via setText).
  • Reused existing upsert flow so missing pads are still created from request body for both methods.

• Server implementation

  • Refactored route logic into a shared request handler with a method-specific mode:
    • POST → API.setText(...)
    • PATCH → API.appendText(...)
  • Preserved existing request size limits, pad-id header handling, and error responses.

• Regression coverage

  • Added frontend tests for:
    • PATCH /post appending to an existing pad.
    • PATCH /post creating a new pad when the target pad does not exist.

• Docs

  • Updated README usage to document PATCH /post and include a curl example.

# Replace pad content
curl -X POST -d @datafile.txt -H 'X-PAD-ID: mypad' http://localhost:9001/post

# Append to existing pad (or create if missing)
curl -X PATCH -d @datafile.txt -H 'X-PAD-ID: mypad' http://localhost:9001/post