docs(webui): document secure workflows and bundled distribution

## Parent / dependencies

Parent: #1834. Depends on #1836, #1838, #1837, #1844, #1845, #1846, #1847, #1848. Documentation may be drafted earlier but must reflect measured final behavior.

## Ownership

Own user/developer WebUI docs and translations in the project's existing layout, README/CLI help links, `docs/webui/README.md`, compatibility manifest final review, NOTICE/release guidance and packaging verification documentation. Coordinate CLI help edits with startup owner; no new website framework or independent package version.

## Required content

- Copy/paste quickstarts for `mlxcel-server --webui`, equivalent `mlxcel serve --webui`, explicit `--models-dir models/mlx`, managed model-store root, single-model mode, offline use, enabled live settings/slots/metrics and safe authenticated remote/proxy access. Explain generated local key/no-terminal behavior and memory-only browser login; never show a real secret or token-bearing URL.
- Explain supported versus downloaded versus loaded, source precedence/containment, explicit preload exception, model/task capability gates, load/drain/memory-release distinction, cancellation limits, cache removal versus unload and capacity/eviction confirmation. Document restart/failed download recovery and non-chat API task examples.
- Separate browser preferences/history, request sampling, live settings partial application, next-load profiles and restart-only controls. Describe actual context/metric scopes and unknown states; link related #1815 without promising unmerged behavior. State privacy defaults, local history opt-in/export/clear, attachment limits, no remote telemetry and no tool execution.
- Contributor workflow: contract-first changes, shared module ownership, frontend source versus generated assets, pinned pnpm install/build, asset regeneration/drift check, offline Rust builds, UI Cargo feature graph and browser/hardware test evidence. Explicitly distinguish `webui/` from project marketing `webpage/`.
- Verify supported release packaging includes embedded UI through the chosen default feature in Metal/CUDA artifacts and no-default-features builds omit it. Retain all frontend/adapted-code licenses in NOTICE and per-file derivation notices; do not redistribute Apple design-kit or system font assets without permission.

## Acceptance criteria

- [ ] English and Korean user docs/help examples are exercised against the installed artifact, not only reviewed as prose; bad root, unauthenticated access and UI-not-compiled diagnostics match docs.
- [ ] b10621 UI alias/disable/environment/static-path/config/tool/proxy classifications accurately describe implemented versus rejected behavior and preserve root-health divergence notes.
- [ ] Bundle source/hash/size, offline moved-binary check and release feature inclusion are linked to final integration evidence. Normal MLX runtime dependencies remain explicit.
- [ ] Final epic checklist links each child/PR/evidence and clearly lists deferred private Hub auth, native packaging, tools/MCP and other task UIs; nothing deferred appears as a functioning button.

Start with docs/webui/README.md and the existing README/CLI help layout, then review the dependent issues and final integration evidence before drafting. Exercise the English and Korean examples against the installed artifact, verify bundled and no-default-features packaging, and document only behavior confirmed by those checks. Keep webui/ documentation distinct from the marketing webpage/.

Written by the indexing model from the issue text.