✨(frontend) introduce performance mode with auto-detection and telemetry

Provide CPU relief during video meetings for low-spec and constrained
devices by dynamically reducing video quality.

Context and motivation:

Video processing can be intensive on resource-constrained hardware.
To ensure accessibility on any device, this feature introduces both
automatic, proactive CPU-constraint detection and manual controls in
settings, so users can reduce CPU load while keeping control over
their experience.

Key changes:

* Performance mode store
  - Central, non-persisted store as the single source of truth for
    performance mode.
  - Decouples inputs (e.g. `CpuConstrainedObserver`, settings
    toggle) from effects (`PerformanceModeController` for publishing
    encodings, `VideoResolutionSubscription` for incoming quality
    caps).
  - Intentionally session-scoped to prevent sticky low-quality
    modes across meetings.

* Auto-detection and user toast notification
  - Automatically activate performance mode when the encoder
    reports CPU constraints.
  - Show a 30-second toast letting users override ("keep my
    quality"), which restores encodings and suppresses further
    auto-activations for the session.

* Video settings tab
  - Update the Videos tab in the settings so it reflects the
    current performance mode state and lets users toggle it
    manually.

* Hardware telemetry snapshot
  - Add a non-throwing, best-effort environment snapshot (core
    count, memory, battery status) attached to performance events,
    to help analyze hardware patterns across sessions.

CHANGELOG.md

@@ -8,6 +8,10 @@ and this project adheres to
 
 ## [Unreleased]
 
+### Added
+
+- ✨(frontend) introduce performance mode with auto-detection and telemetry
+
 ### Changed
 
 - 🔥(frontend) drop unused vendored ConnectionObserver