feat: model download & management UX (P13f-1)

[blokzdev]

P13f-1 — Model download & management UX

First of three P13f phase-close PRs (split confirmed: f-1 model UX · f-2 translation settings surface · f-3 docs phase-close). Exploration found gating is already consistent (shared aiSummaryAction copy; tier-ineligible features hide; the d-3 low-tier path is wired to the "most relevant items" screen) — so the real polish is model download/management legibility.

Every-device-appropriate, settings-screen UX. No schema, no deps.

What changed

• ModelDownloadService — installedModelIds() (cheap existence scan of the models dir) + delete(modelId) (frees a cached model; no-op if absent). Reuses the existing hash-checked isInstalled.
• downloadedModelIdsProvider (new) — drives the tile state; invalidated after download/delete.
• ai_settings_screen.dart generation + transcription model tiles:
  • Subtitle now reads its state: Active · Downloaded · ~XXX MB.
  • A "Delete download" overflow action for downloaded-but-inactive models (frees space; the model stays selectable and re-downloads on demand). The active model has no delete — avoids breaking the active feature.
  • Inline determinate progress during download via the engines' already-exposed downloadModel(onProgress:) (replaces the opaque "Downloading…" snackbar).
• Aligned the one outlier gating string ("Translation isn't available here" → "…on this device") with the established tone.
• The embedder (Gecko floor / multilingual) is intentionally excluded from delete — deleting the active embedder would break semantic search (deferred to BACKLOG with a guarded "switch to Gecko first" flow).

Tests

• model_download_service_test — installedModelIds (lists dirs with files, ignores empties; empty when none), delete (removes the dir, no-op when absent, flips isInstalled).
• ai_settings_screen_test — a downloaded-inactive model shows "Downloaded" + a delete affordance.
• Full suite green (933), dart format + flutter analyze clean. (downloadedModelIdsProvider is a hand-written FutureProvider — no codegen.)

Docs

• P13-PLAN.md — P13f card expanded to the f-1/f-2/f-3 breakdown; f-1 [~] + as-built. VERIFICATION.md — P13f-1 row. BACKLOG.md — embedder-delete + item-detail transcribe-flow inline progress deferred.

Verification

• ✅ CI gates green locally.
• ⏳ APK spot-check owed: tile state reads correctly; downloading shows inline progress; delete frees space and the model re-downloads on next select.

Next: f-2 (translation settings surface), then f-3 (P13 phase close — markers, summary, consolidated cross-feature checklist, and the CLAUDE.md §7 phase-close convention).

https://claude.ai/code/session_013JoYmLCosYt5tQ8qwdbL1T

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