feat(chat): make assistant message text copyable and selectable

[omgoshjosh]

Assistant prose currently has no copy path at all on Android.

Why

src/components/markdown/Markdown.tsx defines a CustomRenderer whose whole purpose is to strip the selectable prop that react-native-marked hardcodes on every plain-text node, because a selectable <Text> inside a FlatList row hits facebook/react-native#46999 on Android. Chat messages are rows of the session screen's inverted FlatList, so every markdown node hits it.

That workaround is correct as far as it goes. But its stated justification —

Code content is still copyable via CodeBlock's explicit Copy button, so dropping selectable on plain text costs little.

— undercounts the cost. Current coverage:

Content	Copyable?	How
User message text	Yes	<Text selectable> in MessageBubble
Fenced code blocks	Yes	CodeBlock's Copy button
Tool call output	Yes	<Text selectable> in ToolCallCard
Assistant prose	No	—

So the one thing users most want to copy — the assistant's actual answer — can't be selected, copied or shared by any means.

Approach

Rather than re-enabling selectable inside the FlatList row (exactly what RN#46999 punishes), add a copy path that reads the source text from the message parts:

• message-copy-text.ts — pure extractCopyText / extractReasoningText / hasCopyableText over a message's parts. Dependency-free, unit-tested.
• SelectableTextModal — renders that text in a selectable <Text> inside a <Modal>. The modal is the point: it renders into its own host view outside the transcript FlatList, so RN#46999 does not apply and real partial selection works.
• MessageBubble — long-press enabled for both roles (was user-only).
• session/[id].tsx — the action sheet offers "Copy message" and "Select text" for either role; "Edit message" stays user-only since reverting to an assistant message isn't supported. Returns early when a message has no prose, so tool-only messages don't open an empty sheet.

i18n strings added to both en and zh-Hans (catalog-parity test enforced).

The second commit fixes a safe-area bug in the new modal: under edge-to-edge the sheet extends beneath the navigation bar, so a fixed paddingBottom left the hint drawn behind it. It now pads by the real bottom inset.

Verification

Verified on an Android 12 emulator against a live server:

1. Long-press an assistant message → "Message actions" sheet with Copy message / Select text (and correctly no "Edit message")
2. Select text → modal shows the markdown source
3. Long-press inside the modal → selection handles plus Android's native Copy / Share / Select all menu

Partial selection of assistant prose now works where it was previously impossible.

tsc --noEmit clean; full suite passes (294 tests) on this branch.

Note

app/demo.tsx renders MessageBubble without onLongPress, so the demo conversation is still uncopyable. Deliberately left out to keep this reviewable — happy to add it here or in a follow-up, whichever you prefer.

[omgoshjosh]

Folding this into #182 to keep the tracker tidy — same commit, same verification, just consolidated with the other two Android fixes rather than three separate PRs. Sorry for the noise.