feat(remap): add multimedia button actions

[blak0p]

Closes #78

Summary

• Adds the eight capture-backed multimedia remap actions for eligible X6 buttons.
• Presents Basic and Multimedia choices while preserving explicit apply/discard behavior.
• Archives the completed OpenSpec change and syncs its canonical specification.

Changes

File	Change
internal/protocol/x6/remap.go	Defines and validates the closed multimedia action set without changing report or ACK handling.
internal/desktop/service.go	Publishes the ordered Basic-plus-Multimedia action catalog.
frontend/src/components/panels/*	Renders grouped actions and prevents Multimedia selection for Button 1.
openspec/**	Archives the SDD change and syncs the canonical remapping specification.

Hardware impact

No live hardware, hidraw node, or device write was executed. The existing 59-byte report and ACK contracts are regression-tested; this change only expands capture-backed action values through existing remap handling.

Test Plan

• git diff --check
• go test ./...
• go test -race ./internal/desktop/...
• go vet ./...
• (cd frontend && npm test) — 8 files / 81 tests
• Existing production builds passed before the archive-only documentation step.

Delivery note

The native reviewer rejected its own capture binding. The maintainer explicitly elected to deliver this candidate without native review.

Size exception

Size exception: 893 changed lines; 472 code/test lines are within the prior 550-line exception, and the remaining lines archive required SDD history.