fix(auth): keep selected provider visible

[rochecompaan]

Summary

• derive the auth provider selector's eight-row window from the selected filtered-list index
• keep long-list, wraparound, and filtered navigation selections visible, including the reported item 14 of 39 regression
• add committed actual-CLI visual evidence for the scrolled selector state

Validation

• node --test src/cli/commands/init/pi-auth-provider-state.test.ts src/cli/commands/init/pi-auth-selector.test.ts — 16/16 passed
• npm test — 1279/1279 passed
• npm run lint — passed
• npm run build — passed
• Prettier, whitespace, dependency-metadata, visual-artifact, and clean-worktree checks — passed

Reviews

• Final Codex full-worktree review: no Critical or Important findings; one deferred P3 point-in-time spec-status note
• Final thermo-nuclear full-worktree review: passed with no findings
• Final validation-readiness review: all required commands passed

Visual evidence

• docs/screenshots/auth-provider-selector-scrolling.gif — actual patchmill auth provider selector with item 14 visible and selected in the current 38-provider runtime catalog; automated coverage retains the exact reported 14/39 case

Closes #170

Patchmill run cost

Stage	Model	Prompt tokens	Output tokens	Estimated cost (USD)
Planning	gpt-5.6-sol	1,030,094	14,631	$1.2994
Development environment	gpt-5.6-sol	493,208	3,909	$0.8323
Implementation	gpt-5.6-sol	6,678,795	46,411	$6.7432
Implementation	gpt-5.6-terra	3,258,462	9,782	$1.0471
Implementation subtotal		9,937,257	56,193	$7.7903
Total		11,460,559	74,733	$9.9219

Prompt tokens include uncached input, cache reads, and cache writes. Cost is an estimate based on Pi's recorded model pricing and includes parent and subagent sessions.