fix: phone touch targets, send and stop names, and the Flow caret

[divyanshub024]

Summary

Mobile fixes from a pass over the example on the iPhone simulator. Stacked on #35 (shared changelog); merge that first.

• Touch targets. Message actions sit on a 20px frame, the attach, stop, remove and error-dismiss discs on 24 to 32px, the menu triggers on 32, the code block's copy on 24. A finger needs 44 (Apple) to 48 (Material). New FlowTouchTarget widens the hit area on touch platforms without changing layout, sized per site so a neighbour's taps stay its own (siblings hit-test in reverse order, not by distance; the actions row takes its reach downward into the gap between turns). Pointer platforms pass straight through.
• Send and stop names. The discs had no accessible name; VoiceOver announced an unnamed button. FlowComposer.sendTooltip and stopTooltip name them, as tooltips, which carry the semantics label. Docs and example updated.
• Caret and selection colour. Nothing set them, so a host installing FlowTheme on a stock ThemeData got Material's purple caret, wash and iOS handles against Flow's primary. New FlowSelectionTheme derives TextSelectionTheme and the Cupertino primary from FlowColors; wraps the chat view and the composer.
• The example gates its web-only drop and paste callbacks with kIsWeb, which silences the two runtime warnings it logged on iOS.

Screenshots

Before	After
Purple caret in the composer (screenshots attached separately)	Caret in the Flow primary

How this was verified

• iPhone 17 Pro simulator, example app, light and dark: caret colour after hot reload; layout unchanged around the message actions, composer row and code block header; XXXL Dynamic Type still lays out.
• flutter analyze clean at the root and in example/ and playground/; dart format . applied.

Checklist

• flutter analyze lib and flutter analyze in example/ and playground/ are clean
• dart format . applied
• Exercised in the playground — with a stage demo added or updated if this is a new component or variant
• Any new entry under dependencies: in pubspec.yaml is flutter.dev-published, forces no configuration on hosts that never use the feature, and is argued in this PR
• Nothing model-facing — no prompts, schemas, or provider/network calls
• New public API is exported from lib/flow_ui.dart and documented in docs/ and the README table
• CHANGELOG.md updated for user-facing changes, with breaking changes called out
• PR title follows conventional commits (feat:, fix:, refactor:, docs:, chore:)

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Note

Low Risk
Localized UI/a11y and layout hit-testing on touch; no auth, data, or breaking API beyond optional composer tooltip params.

Overview
Improves mobile usability and accessibility without shifting desktop layout: new FlowTouchTarget (iOS/Android only) reserves larger hit boxes in layout and forwards taps to small controls—message actions, composer attach/send/stop rings, menu triggers, code-block copy, and error dismiss—often by reclaiming footer/row gap so pixels stay on-spec.

Adds FlowComposer.sendTooltip and stopTooltip so VoiceOver gets names for send/stop (documented in composer docs; example sets 'Send' / 'Stop'). FlowSelectionTheme wraps the chat surface and composer so caret, selection wash, handles, and Cupertino primary use Flow primary instead of the host ColorScheme purple.

The example passes kIsWeb-gated drop/paste handlers to avoid iOS runtime warnings. CHANGELOG.md records touch targets, tooltips, and the caret fix.

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[divyanshub024]

Folded into #37, which now targets main with these commits unchanged.