[JFWooten4]

Problem

Clicking the Sweetie Bot Summarize with ChatGPT icon on a large local PDF can leave a blank popup, followed by a long gap before ChatGPT displays its own upload indicator. It looks as though nothing is happening even while the existing attachment handoff is preparing document bytes.

Change

• Replace the temporary about:blank popup/tab with an immediate Preparing your PDF… screen, including an accessible loading state and reduced-motion support.
• Mark the viewer's summary button busy while launching.
• Display a small ChatGPT-side status from page initialization through attachment preparation, internal transfer percentage, ChatGPT upload acknowledgement, and final submission. Percentages describe only the acknowledged PDF byte transfer, not ChatGPT's own upload progress.
• Surface attachment failures with a clear manual-upload reminder and preserve the existing unsent-draft fallback.
• Extend popup regression tests to cover the new preparation screen and busy-state cleanup.

Context

Builds on the local PDF attachment handoff in #196. The existing flow continues to use the PDF.js data already loaded in the viewer and relays chunks in memory; this PR does not introduce another PDF upload service or persistent PDF storage.

Validation

• Verified the proposed Git tree differs from main in exactly four intended paths: the new preparation page, viewer handoff, ChatGPT content script, and popup tests.
• Confirmed the branch is one commit ahead of main, with no unrelated changes.
• Updated automated popup test assertions. Tests were not executed locally, because a runnable repository checkout is unavailable in this environment.
• Browser handoff not live-tested against ChatGPT's current UI; its file input and attachment acknowledgement are frontend-dependent.