mesh,web: add central event log for GenericEvent history

[Variour]

Summary

Closes #442.

• Adds EventLog (src/events/EventLog.h): a fixed-size (128-entry, ~9.5KB) RAM ring buffer recording GenericEvent broadcasts (e.g. buzzer presses/resets) in receipt order, with sender name resolved via PeerRegistry::nameFor (new).
• Adds GET /api/events?eventType= to read the log, optionally filtered by event type, capped to the eventLogLimit device setting (default 10, most-recent-first).
• Adds POST /api/events/clear to wipe the device's ring buffer and restart arrival order at 0, broadcasting t:'eventsCleared' so other open viewers stay in sync.
• Adds a persisted eventLogLimit device setting (1–128, default 10) controlling how many of the most recent matching entries the endpoint above returns — configurable from Settings, applied live (no reboot).
• Pushes new entries live over the existing WebSocket as a t:'event' message, alongside t:'log'|'peers'|'groups'.
• Adds a read-only "Event log" card to the web UI (Settings tab): a "Max displayed" field, an eventType filter, a table of name/eventType/payload/arrival order updated live via the WebSocket push, and a confirm-gated "Clear event log" button.
• Mirrors the endpoints, WS messages, and static sample data in the mock server (server/index.js) per the repo's device/mock/UI parity checklist.

Deviation from the issue's design: the issue calls for registering /api/events in the shared (ApiRequest, ApiResponse) dispatch table introduced by #436, so it's automatically reachable over serial. #436 is still an open, unmerged PR, so this endpoint (and the new clear endpoint) are registered directly against AsyncWebServerRequest for now (matching every other current endpoint) — they should move onto the shared dispatch table once #436 lands, at which point they'll also become serial-reachable.

Test plan

• npm test (mock server test suite, 36/36) passes
• npm run lint (ESLint) passes
• clang-format --dry-run --Werror passes across the touched src/ files
• Verified via Playwright against the mock server: initial GET /api/events render, eventType filter narrowing the list (including the empty-result case), changing/saving eventLogLimit and confirming the backend persists and applies it (most-recent-N slicing verified after reload), and the clear button (confirm dialog, backend ring buffer actually emptied, cancel path leaves everything untouched) — no console errors
• Verified the live-push paths (_onEventPush, eventsCleared handler) by direct review, since the mock server's dashboard WebSocket is intentionally never opened when served from localhost (pre-existing behavior, see data/index.html:3010)
• Not verified in this environment (no physical device): real mesh GenericEvent broadcasts arriving and being recorded/pushed end-to-end on hardware

[github-actions]

Preview

URL: https://batterylight-pr-451.blackpond-ef672c92.germanywestcentral.azurecontainerapps.io

Hosted on Azure Container Apps — accessible from the internet.