feat(voice): add a Voice settings page with provider choice and a local model catalog

Each host gets a Voice page under host settings. Dictation and voice mode
each pick a provider (Local or OpenAI) and a model. Provider settings sit in
one group of collapsible rows: local speech-to-text and text-to-speech list
every catalog model with its size, languages and description before download,
and OpenAI takes a write-only API key and an optional base URL.

The local catalog grows from Parakeet/Kokoro to Whisper, SenseVoice, Dolphin,
GigaAM, Parakeet Japanese, multilingual Kokoro, Kitten and Piper voices. Each
entry declares its sherpa engine family, so the recognizer and TTS build the
matching model config and the downloader verifies the files that family needs.
Kitten v0.8 is left out: it crashes the pinned sherpa-onnx 1.12.28.

Voice mode is now opt-in. The default lived only in code (true), never in
config.json, so hosts that never toggled it find it off after upgrading.

OpenAI speech-to-text resolved one model for both features, with voice mode's
choice taking precedence, so dictation silently used voice mode's model. The
resolved config now carries a model per feature; STT_MODEL still overrides both.

Rows revealed under a collapsible settings row are a nested density level set
by the container: tighter padding, smaller text, and xs controls, with
controls taking their size from the row instead of each caller.

docs/design.md

@@ -70,7 +70,7 @@ the sidebar. Apply it as `backgroundColor`; setting `opacity` on the control als
 
 Sizes: `xs` for ultra-tight inline triggers. `sm` for any button sitting in a row. `md` is the page default. `lg` is reserved for large standalone CTAs.
 
-Sizes are a shared contract across control kinds, defined once in `control-geometry.ts`: `xs` = 28px tall with `fontSize.sm` labels, `sm` = 32px with `fontSize.base`, `md`/`lg` = 44px with `fontSize.base`. `<SegmentedControl>` (`packages/app/src/components/ui/segmented-control.tsx`) takes the same `xs`/`sm`/`md` sizes — a segmented control next to a `<Button>` of the same size always matches in height, label size, and corner radius. Its segments run one padding step tighter than a button, because the gap between segments already reads as padding. The selected segment is a `surface3` fill with `foreground` text, not an inverted one — inverting it inside thin chrome puts a white slab in the toolbar. Thin chrome such as the file toolbar uses `xs`; settings rows use `sm`. Never shrink a control's font or padding locally to fit a context — if the context needs a smaller control, the size tier is missing or the wrong one is in use.
+Sizes are a shared contract across control kinds, defined once in `control-geometry.ts`: `xs` = 28px tall with `fontSize.sm` labels, `sm` = 32px with `fontSize.base`, `md`/`lg` = 44px with `fontSize.base`. `<SegmentedControl>` (`packages/app/src/components/ui/segmented-control.tsx`) takes the same `xs`/`sm`/`md` sizes — a segmented control next to a `<Button>` of the same size always matches in height, label size, and corner radius. Its segments run one padding step tighter than a button, because the gap between segments already reads as padding. The selected segment is a `surface3` fill with `foreground` text, not an inverted one — inverting it inside thin chrome puts a white slab in the toolbar. Text fields take `xs`, `sm` and `md`. Thin chrome such as the file toolbar uses `xs`. A settings row sizes the controls in its trailing slot itself, `sm` in a card and `xs` when nested (`packages/app/src/components/ui/control-size.tsx`), so don't pass a size to a button, dropdown or field placed in a row. Never shrink a control's font or padding locally to fit a context — if the context needs a smaller control, the size tier is missing or the wrong one is in use.
 
 A `<Pressable>` wrapping a `<Text>` is a sixth variant. It is wrong. `<Button>` accepts `style`, `textStyle`, `leftIcon`, `disabled`, `size`, and `variant`.
 
@@ -126,6 +126,8 @@ Cards inside a section sit closer than sections. Rows inside a card touch — on
 
 Rows have generous vertical padding: roughly 16px of content plus 16px of vertical padding for settings rows, 8–12px for sidebar list items where many rows must fit. Compressing rows below the established density to fit more on the screen is wrong. Too many rows means more cards or more sections, not smaller rows.
 
+Rows revealed under a `<SettingsCollapsibleRow>` are nested rows, one step smaller than card rows: 8px of vertical padding, `fontSize.sm` text, `xs` controls, and one fixed height so rows with and without a trailing button line up. The collapsible row sets this for everything under it (`packages/app/src/components/settings/row-density.tsx`). Never pass a density to a row, and never pad or resize a nested row by hand.
+
 The whitespace is the design.
 
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