fix(e2e): harden activation-e2e newer flows (issue #104) (#105)

Root-caused directory-picker's directory-row-frontend failure: all four
in-app @gorhom/bottom-sheet sheets (DirectoryBrowserSheet, DirectorySwitcher,
ModelPicker, VariantPicker) provide static percentage snapPoints but rely on
v5's enableDynamicSizing default (true), which never resolves without content
wrapped in a size-reporting component — so useAnimatedDetents() permanently
early-exits and the sheets can never actually open. Set
enableDynamicSizing={false} on all four (they already have explicit
snapPoints, so dynamic sizing was never needed).

variant-picker's chip failure was a stale test assumption: src/lib/
model-selection.ts's chooseModelSelection() deliberately returns null for a
fresh session (issue #37/#35 — the provider registry default is unreliable),
so a brand-new session has no model selected and the reasoning-effort chip
has nothing to key off of. Added testIDs (model-chip, model-option-*) and
updated the flow to explicitly pick a model first, matching real usage.

diff-scroll's missing markdown text: switched src/components/markdown/
Markdown.tsx from react-native-marked's FlatList-based default export to its
useMarkdown() hook rendered into a plain View. The chat screen already nests
this inside its own *inverted* FlatList (one row per message) — a nested
VirtualizedList inside an inverted outer list is a known RN footgun where the
inner content can render at zero height instead of just warning. We already
forced scrollEnabled:false + a large initialNumToRender, defeating
virtualization anyway, so rendering the parsed blocks directly loses nothing.
Extended the existing react-native-marked .d.ts shim (added for a React
18/19 ReactNode mismatch) to also declare useMarkdown/useMarkdownHookOptions.

Added diagnostic screenshots to directory-picker.yaml and diff-scroll.yaml
at the previously-failing steps for faster triage if these regress again.

Closes #104.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

src/components/chat/VariantPicker.tsx

@@ -50,6 +50,9 @@ export function VariantPicker({ variants, selected, isDark, onSelect, sheetRef }
       ref={sheetRef}
       index={-1}
       snapPoints={["30%", "50%"]}
+      // See DirectoryBrowserSheet.tsx for why this is required alongside
+      // static snapPoints (issue #104): without it the sheet can never open.
+      enableDynamicSizing={false}
       enablePanDownToClose
       backgroundStyle={isDark ? s.sheetDark : s.sheet}
       handleIndicatorStyle={{ backgroundColor: isDark ? "#666666" : "#cccccc" }}