webui: stop overriding engine and package defaults from model_params.json

[CryptVenture]

Split out of #374 as requested: UI defaults that override the engine or the package. The dead controls, the range fixes, the missing groups and the label text are separate PRs.

The problem

Every default in model_params.json is sent on every request. A value here is not a fallback the engine can improve on — it is an override that wins over the model's own default and over anything the package ships.

Group / control	UI sent	Engine or package value	Code
vibevoice.num_inference_steps	10	ddpm_num_inference_steps from the package, 20 in the shipped GGUF	src/models/vibevoice/assets.cpp:112
ace_step.shift	3.0	1.0	include/engine/models/ace_step/types.h:42, applied at request_parser.cpp:231
firered-audio-asr.max_new_tokens	512	300, and 1024 when enable_thinking is set	src/models/firered_audio/session.cpp:162
personaplex.text_top_k	250	follows top_k when unset	src/models/personaplex/request.cpp:111
vevo2.top_k	20	the checkpoint's own generation config	src/models/vevo2/session.cpp:898
fireredtts3-instruct-vdesign.guidance_scale	2.0 (the instruct path's value)	1.2	src/models/fireredtts3/session.cpp:159, 170, 179

vibevoice is the sharpest one: the UI silently halved diffusion quality on every request.

The change

Four defaults are omitted rather than corrected, because the right value is not a constant — it is read per package or derived from another option, and any literal here would override it again. Omitting a default is how this file says unset: the option parser skips empty values, so the control renders blank and sends nothing until the user types something.

Two defaults are corrected to the engine's value (ace_step.shift, the voice-design guidance_scale).

Validation

python3 -m json.tool webui/configs/model_params.json   # parses
python3 tools/check_loader_catalog_sync.py
# ok: runtime loaders, model_specs, and model_manager_v2 are in sync

Each value was read from the source line named beside it. These families have no package installed on this machine, so they were not run end to end; a live server was used to confirm the empty-default behaviour renders and sends nothing.

Scope

One data file, six controls. No code change. The generated bundle is deliberately excluded — catalog.ts inlines this file at frontend build time and the bundle is not byte-reproducible, so regenerating it in each PR of this split would make the PRs conflict; happy to send one bundle-regeneration PR once the series lands.

[CryptVenture]

Closing for now to stay inside the 3-concurrent-PR policy (see the discussion on #422). Nothing is wrong with the change and CI is green on it; I will reopen it as review slots free.