Summary

• The upload dropzone (#upload-drop in the Index tab → Upload mode) currently spans the full panel width (~750px on a typical card) with only an icon and two short lines of content. It looks disproportionately empty.
• This PR caps .upload-drop at max-width: 520px with margin: 0 auto, so the dropzone itself centers inside the panel while sibling controls (file list, upload button, result rows) keep their natural full-width layout.
• 520px sits inside the 480–560 range called out in the umbrella issue.

Scope

• One file: packages/memtomem/src/memtomem/web/static/style.css (one rule, two declarations added).
• No JS, HTML, or i18n changes. No behavior change to drag/drop handlers.

Refs

• Umbrella: feat(web): Index tab mid-priority follow-ups (each ships as its own PR) #582 (Prev refactor(stm): decouple surfacing via remote-only MCP #2 — Upload dropzone proportions)
• First of the eight follow-up PRs from that umbrella; the others ship separately.

Test plan

• uv run ruff check packages/memtomem/src && uv run ruff format --check packages/memtomem/src
• Served-asset verification: mm web running locally; curl http://127.0.0.1:8080/style.css?v=68 includes the new max-width: 520px; margin: 0 auto; declarations on .upload-drop.
• Visual check in a browser at the Index → Upload tab (skipped locally — a stale Playwright MCP session was holding the user-data-dir; the change is a 2-line CSS rule with no JS interaction so failure modes are linter/parse-level, all caught above. Reviewer visual check welcome.).

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