Require explicit local AI model names

[cursor]

Summary

Remove hard-coded local model fallbacks for Ollama/vLLM summary and evolution providers. Empty or whitespace-only model settings now produce a direct configuration error instead of attempting a guessed model. The settings UI also stops auto-populating local provider model names and validates that local providers have explicit models.

Fixes ENG-516

Test Plan

• bun -F native test:unit -- ai-provider-validation.test.ts
• bun -F native build
• cargo +stable test --manifest-path apps/native/src-tauri/Cargo.toml test_non_empty_trimmed_string_filters_empty_values (blocked by missing system library gdk-3.0 required by Tauri on this Linux runner)

Docs

• Docs updated (companion PR in darkmatter/nixmac-web: #___)
• No docs update needed

  

[github-actions]

	Warnings
⚠️	PR is marked WIP / draft — do not merge until ready for review.
⚠️	Please assign this PR to someone (usually yourself).

	Messages
📖	No docs update needed — acknowledged.

📋 PR Overview

Lines changed	152 (+119 / -33)
Files	0 added, 7 modified, 0 deleted
Draft / WIP	yes
Has Test Plan	yes
New UI components	no
New Storybook stories	no
New Rust modules	no
New TS source files	no
New tests	no
package.json touched	no
Cargo.toml touched	no
Infra / CI touched	no

🔬 Coverage

Report	Lines	Statements	Functions	Branches
apps/native/coverage/coverage-summary.json	18.0%	18.0%	31.0%	54.3%

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