feat(desktop): HA value-setting slider for #442 Slice 1

[badbread]

Summary

Part of #442

Desktop client (Flutter) implementation of #442 Slice 1's HA value-setting controls (dimmable light brightness, cover position, fan speed), consuming the backend contract frozen in #460 (still open, not yet merged into main — the contract's shape was confirmed directly against that branch's services/api/src/ha.rs).

• lib/api/ha_models.dart: new HaControlDescriptor (action/kind/value/min/max/step/unit), parsed defensively off HaEntityState.control — any missing/malformed field yields null rather than throwing, so an older server (or an odd entity) never breaks the whole /ha/states decode.
• lib/api/ha_api.dart: haAction() gains an optional value param, sent only when non-null.
• lib/ui/ha_overlay/ha_actions.dart: haValueActionLabel (row caption), haValueActionNeedsConfirm (cover/lock parity with the discrete button confirms), haFormatControlValue (kind-agnostic display, e.g. "62%").
• lib/ui/ha_overlay/ha_state_card.dart: a value row (Slider + caption) below the existing button row, shown only when the host passes a control descriptor. Bounds/step/unit come entirely from the descriptor — never hardcoded — so Slice 2's climate.set_temperature can reuse the same widget. Commits on release with exactly one POST per drag gesture (never while dragging), never optimistically flips the displayed value (same settle-window/in-flight-spinner convention as the button row), and confirms first (with the target value in the prompt, e.g. "Set Garage Door to 20%?") for cover.set_position or a require_confirm link.
• lib/ui/ha_overlay/ha_overlay_layer.dart: gates the slider like the button row (actuators capability + actuator role + the link's allowed_actions), wires it to POST /ha/action, and adds a right-click (secondary-tap) gesture on any controllable badge to open the detail card. This was needed because a controllable "simple" domain (light/fan) already direct-fires its primary action on left-click and never reaches the card (issue HA controls: single-click actuates directly; hover shows state; card only for multi-action devices (#187 follow-up) #428) — so a dimmable light had no gesture to reach its slider. Left-click behavior is unchanged.
• lib/ui/overlay_editor/overlay_editor_layer.dart: adds an optional onSecondaryTapItem callback to the shared view-mode gesture layer (used only by the HA overlay; the PTZ panel's usage is edit-mode only and unaffected).

Backend, Android, and iOS were not touched.

Verify

• Manual review: null-safety, every referenced symbol exists, imports correct, brace/paren balance checked.
• Caught and fixed two num.clamp() return-type pitfalls (clamp() returns num even when called on an int/double, so Slider's divisions/value need an explicit .toInt()/.toDouble()).
• flutter analyze — BLOCKED: winbuild (10.1.10.90), the only Windows Flutter box, is offline (SSH connection timed out). dev1/dev2/macmini were also checked and have no Flutter toolchain. Did not build/run on this Windows workstation per policy.

Test plan

• Run flutter analyze on winbuild once it's back online; fix any ERRORS.
• Manual smoke test against a server on feat(ha): value-setting HA controls (brightness/position/speed), #442 Slice 1 #460's branch: dimmable light shows a slider on right-click, drag-and-release sets brightness with one POST, cover position slider confirms before sending, fan speed slider respects its percentage_step.