Add priority-only Discord notifications

[HimanM]

What changed

• add persistent Discord webhook settings, strict official-URL validation, CSRF-protected configuration and test endpoints
• send priority-category notifications for new Drops, successful claims, campaign completion, mining lifecycle changes, and channel availability problems
• format messages with category artwork, reward thumbnails, campaign details, timestamps, mention suppression, deduplication, and Discord rate-limit retry
• add a responsive Settings card with masked webhook state, event toggles, test delivery, and removal
• fix the first mobile overview metric so its separator rail and padding match the other metrics

Why

Server users need low-noise Discord updates without changing the core miner. All category-scoped events are filtered against the priority list, and operational alerts are separately opt-in.

The mobile metric mismatch came from a first: exception that removed the first metric's separator and padding while every sibling retained them.

Persistence and security

• webhook configuration and notification deduplication live in the existing update-safe web-auth.sqlite3
• the full webhook URL never returns to the browser
• existing browser sessions, Twitch cookies, and miner settings remain unchanged across installer updates
• no core miner or installer script files changed

Validation

• python -m unittest discover -s tests (68 tests)
• npm run build
• git diff --check
• repository scan confirmed the user-provided webhook token is absent