feat: group handling (WebUI + NVS + HA cover entities)

[manuschillerdev]

Goal

User can define groups of blinds, persist them in NVS, and control them as a single cover entity in Home Assistant.

Depends on #41 (TX multi-dest).

Design

What is a group?

A named set of device references (by address or slot) that share "open/close together" behavior. Stored in NVS. Exposed as a single cover entity via MQTT / native API.

Native multi-dest vs. sequential TX

If all members share src_address (same emulated remote), emit one multi-dest 0x44 packet (#41). Otherwise fall back to sequential single-dest commands. Detect at group creation time, cache the decision.

Position / state derivation

Group position derived from member states (e.g. average of member positions). Group state: if any member is moving, group is moving.

Changes

1. Group definition in NVS

Group NVS struct (or reuse slot system with DeviceType::GROUP). Each group stores:

• Name
• List of member addresses (up to 10)
• src_address (shared remote address for native multi-dest)

2. DeviceRegistry group CRUD

• add_group(name, member_addresses[]) -> Group*
• remove_group(address)
• command_group(group, command_byte) — calls multi-dest TX or falls back to sequential

3. MQTT adapter: group cover discovery

Publish HA MQTT discovery for each group as a cover entity.

• state_topic derived from group address/name
• command_topic routes through DeviceRegistry::command_group()
• Position support only if ALL members have durations configured

4. Web UI: group management

• CRUD for groups (create, rename, delete, add/remove members)
• Display group state derived from member states
• Send group commands (open/close/stop)

5. Native API adapter

Expose groups as ESPHome cover::Cover entities for native mode (if applicable).

Validation

• Create a 2-blind group in NVS, verify MQTT discovery
• Send open/close via HA, verify both blinds move
• Verify position updates in HA reflect member states
• Remove one member, verify group still works

Not in scope

• "all" broadcast — just a group containing all blinds
• Tilt support for groups (revisit later)