Release 0.7.3 — scrub all remaining Nexus mentions

[HLLMR]

Cuts 0.7.3 and completes the Nexus removal.

#151 removed the Nexus source integration (app + landing). This PR is the clean sweep — every remaining tracked mention of Nexus is gone:

• Public website: the four legal pages (.md + .html), the trust page, llms.txt, og.svg, screenshots README — including removing the obsolete "Nexus API key" privacy/terms clauses.
• App: the tauri CSP (dropped *.nexusmods.com), the referer-host list, Rust comments, a test example URL.
• Docs/marketing: README, SECURITY, .github release notes, all docs/social/* and docs/* (source lists, "endorse on Nexus", the "Nexus Mod Manager" naming note).
• Housekeeping: removed the dead nexus-upload/ gitignore entry and old CHANGELOG feature phrasings.

The only Nexus mention left in the repo is the CHANGELOG [0.7.3] "Removed" entry — the public paper trail:

Nexus Mods integration has been removed entirely. … Nexus support flagged our SiloAPI index model as an AUP violation on a second review — after approving the same approach on the first — so rather than keep rebuilding toward a moving compliance target for a minor Farming Simulator source, we pulled Nexus out completely.

Verification: git grep -i nexus → CHANGELOG.md only.

Gate: npm build ✓ · fmt ✓ · clippy -D warnings ✓ · 98 tests ✓ · audit exit 0.

(SiloAPI's ingest/DB purge is infra's, in progress separately.)