webui: give the parameter controls English labels and accurate help text

[CryptVenture]

Split out of #374 as requested: label and help text only. The dead controls, the default overrides, the range fixes and the missing groups are separate PRs.

The change

• 42 controls gained label_en / info_en / placeholder_en. They carried a Chinese label with no English counterpart, so the English UI fell back to printing the raw option key.
• Several info strings described behaviour the engine does not have and now describe what it does.
• The file's own _comment claimed only user-modified values are sent. That has never been true in this UI — every control with a default is sent on every request — so the comment now documents the real behaviour, including that omitting a default is how a control says unset (the option parser skips empty values).

No name, type, default, minimum, maximum, step or choices value changes here, so no request shape changes.

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

Scope

One data file, 43 lines. No behaviour change of any kind. 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.