Add encoder map page

[arakashic]

Summary

• New Encoder tab: per-direction keycode display and remapping for boards with encoders.
• Uses standard XAP routes (KeymapGetEncoderKeycode / RemappingSetEncoderKeycode); no new XAP capabilities required.
• Bulk encoder_keymap_get Tauri command replaces N×M×2 individual round-trips on page mount; logs fetch_encoder_keymap total + per-call timing.
• Reuses the keymap KeycodePicker (from Keycode sync pipeline and data-driven keymap picker #19) for assignment, including parameterized templates.

Dependencies

Depends on #19 (Keycode sync pipeline and data-driven keymap picker). Merge order: #19 → this.
Until #19 merges, the PR diff shows the keycode-pipeline commits too; they drop out automatically after #19 lands.

Test plan

• cargo build / cargo test / yarn ts-check clean
• On an encoder board, open the Encoder tab; verify per-layer/per-direction display
• Remap a slot via picker; verify display refreshes
• Encoder tab hidden on non-encoder boards

Screenshot