feat: add native PowerStation fan controls

[sknowledge1]

PowerStation has no native client for its proposed fan interface, leaving supported
handhelds without fan controls in the stock quick-access UI. This adds a Fan
Controls card backed by the versioned D-Bus API in
PowerStation PR #54.

Result

• Discover fan objects from /org/shadowblip/Performance/Fan and hide the card
  when PowerStation exposes none.
• Show a selector only when more than one fan is available.
• Display temperature, RPM, effective duty, backend type, configured limits,
  current state, and automatic-fallback faults.
• Provide Automatic, Manual, Curve, and named-preset controls using the atomic
  PowerStation methods.
• Build curve controls from each fan's reported limits instead of embedding
  AYANEO-specific points in OpenGamepadUI.
• Mark a selected preset as Custom as soon as one of its points is edited.
• Use the existing card, slider, dropdown, and FocusGroup patterns for full
  controller navigation.

The client requires fan API version 1 before every hardware-changing call. A
missing, legacy, or future interface leaves the status visible but disables the
controls with an upgrade message. Fan state remains global in PowerStation; it is
not added to OpenGamepadUI's per-game profiles.

Service lifecycle

The PowerStation integration recreates CPU, GPU, and fan resources when the
service becomes available again. Fan signal and refresh-timer connections are
removed when a fan is replaced, so an old object cannot update the active card.
Pending slider edits are retained until the user commits them instead of being
overwritten by telemetry refreshes.

Validation

• cargo fmt --all -- --check
• cargo check --locked -p opengamepadui-core
• warning-strict Clippy, allowing only the two lint classes already present in
  the upstream base
• cargo build --locked -p opengamepadui-core
• Godot 4.7.1 headless import
• focused GUT suite: 7 tests, 22 assertions
• clean git archive rebuild at commit
  c1be4b6348a41722a9290cff33417220dd9906da

The focused suite covers no-fan and multiple-fan visibility, mode mapping,
fallback and API-version status, version-gated writes, preset/custom transitions,
and controller focus. PowerStation PR #54 has completed a controlled real-fan
handoff on the connected AYANEO 3, including Quiet/manual roundtrips and
SIGKILL/watchdog recovery. This OpenGamepadUI branch has not replaced the stock
session on the device; controller/UI acceptance remains a packaged testing-image
gate. The device was returned to its rollback service after backend validation.

Addresses #505. The earlier device-specific plugin/store submission was closed
as superseded by this native path.