fix(privacy-filter)!: resolve labels from modelName, fix fromCustomModel identity collapse

[msluszniak]

Description

PrivacyFilterModelSources carried labelNames / viterbiBiases on it but usePrivacyFilter's deps only watched modelName / modelSource / tokenizerSource, so two custom loads with the same sources but different labels collapsed to the same identity and the runner kept the old config silently. fromCustomModel made it worse by hardcoding modelName: 'custom'.

Followed @chmjkb's suggestion on #1116: match the ObjectDetection / Segmentation shape — drop labelNames / viterbiBiases from PrivacyFilterModelSources, resolve them from modelName via an internal lookup, and keep custom-label support only on fromCustomModel (unreachable from the hook). The hook's existing deps now fully cover the prop surface, so the collapse is gone.

Introduces a breaking change?

• Yes
• No

(labelNames / viterbiBiases are no longer accepted as fields on PrivacyFilterModelSources or on the PRIVACY_FILTER_* constants. Custom label spaces and Viterbi tuning move to PrivacyFilterModule.fromCustomModel.)

Type of change

• Bug fix (change which fixes an issue)
• New feature (change which adds functionality)
• Documentation update (improves or adds clarity to existing documentation)
• Other (chores, tests, code style improvements etc.)

Tested on

• iOS
• Android

Testing instructions

1. Run `apps/llm` and open the Privacy Filter screen.
2. Switch between the OpenAI and Nemotron presets — both should load and produce the expected entity spans.
3. Re-trigger the hook with the same preset — should not reload.

Related issues

Closes #1116

Checklist

• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have updated the documentation accordingly
• My changes generate no new warnings