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.toml

@@ -1,6 +1,6 @@
 [package]
 name = "mangalord"
-version = "0.84.0"
+version = "0.85.0"
 edition = "2021"
 default-run = "mangalord"
 
@@ -48,7 +48,7 @@ futures-core = "0.3"
 futures-util = "0.3"
 bytes = "1"
 chromiumoxide = { version = "0.7", features = ["tokio-runtime", "_fetcher-rusttls-tokio"], default-features = false }
-sysinfo = { version = "0.32", default-features = false, features = ["system"] }
+sysinfo = { version = "0.32", default-features = false, features = ["system", "component"] }
 nix = { version = "0.29", features = ["fs"] }
 scraper = "0.20"
 reqwest = { version = "0.12", default-features = false, features = ["rustls-tls", "socks", "cookies", "stream", "json"] }