Simple Weight

- **Test mode persistence:** `/weight testmode` now saves the flag in the player’s persistent NBT data through `WeightManager`. It is restored on login via `WeightDebuffHandler.onPlayerLogin`. It survives relogging and server restarts.
- **Nested backpacks:** `calculateBackpackWeight` is now recursive — it counts the weight of backpacks stored inside other backpacks, for example an L2 backpack inside a Sophisticated backpack.
- **Client config sync on dedicated servers:** added a new `WeightConfigSyncPayload` packet, play-to-client, which sends `item_weights`, `thresholds`, `debuffs`, `multiplier`, and `smooth_transition` to the client on login and on `/reload` through `OnDatapackSyncEvent`.

This fixes HUD and tooltip issues where weight was shown as `1.0` and the bar was not scaled correctly on dedicated servers (Single player too).

## Features

- **Smooth transition:** added a new optional datapack field, `smooth_transition`, defaulting to `false`. When enabled, it linearly interpolates the speed multiplier between the four thresholds instead of applying instant jumps. Implemented in `WeightData.getSpeedMultiplier(weight)`.
- **Tag support:** `item_weights` now accepts `"#namespace:path"` entries, for example `#minecraft:logs`. Tags are expanded into concrete items during data loading through `WeightDataLoader.expandTags`. Item-specific values override tag-based values.