feat: restore panel macros from labeled blockquotes on upload

[blaipr]

Problem

Panels were one-way: download degraded them to labeled blockquotes, but uploading the same file turned them into plain blockquotes, losing the panel styling on every round trip.

Change

md_to_confluence_storage now recognises blockquotes whose first paragraph starts with **Info:**, **Note:**, **Warning:** or **Tip:** (exactly the shape download produces) and restores the matching panel macro, with the remaining blockquote content as the panel body. Labels are matched case-insensitively; any other label (custom panel titles) stays a regular blockquote. Placeholder substitution now runs in reverse insertion order so code blocks and images nested inside a panel body resolve correctly.

Compatibility

Round trips now preserve standard panels. Anyone deliberately writing > **Note:** ... as a plain blockquote will get a note panel on upload — documented in docs/conversion.md.

Checklist

• ruff check src tests, ruff format --check src tests, python -m pytest tests/ pass locally
• Tests added (label matching, case-insensitivity, unknown labels, full round trip)
• docs/conversion.md Panels section updated (no longer download-only)
• CHANGELOG.md updated under Unreleased