feat(admin): overview dashboard sections + system temperature/load sensors

Rework the admin Overview tab into a full-width vertical list of
per-section summary cards (System, Crawler, Analysis, Mangas, Users,
Settings), each linking to its tab. Crawler and Analysis cards update
live over the existing SSE streams; System/Mangas/Users poll; the
Settings strip reflects daemon state. The disk/memory/CPU boxes move
into the System card.

Add a composed GET /admin/overview endpoint returning Users, Mangas and
Analysis aggregates (user counts + newest, manga sync-state counts +
chapter/page totals + newest, library analysis coverage), reusing the
existing MANGA_SYNC_STATE_CASE and the storage handler's try_join fan-out.

Extend the System tab with hardware sensors: CPU load average and
per-core usage (sysinfo), plus temperatures via sysinfo's Components API
(enabled the `component` feature). Sensors degrade to an "unavailable"
state when not exposed (e.g. inside containers without /sys access), and
a temperature at/above its critical threshold raises a warning alert.

Tests: integration tests for /admin/overview and the extended /admin/system
shape; vitest coverage for the new client types and getOverviewStats.
Bump to 0.85.0 (minor) in lockstep.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/Cargo.lock

@@ -1517,7 +1517,7 @@ checksum = "c41e0c4fef86961ac6d6f8a82609f55f31b05e4fce149ac5710e439df7619ba4"
 
 [[package]]
 name = "mangalord"
-version = "0.84.0"
+version = "0.85.0"
 dependencies = [
  "anyhow",
  "argon2",