SwiftUI: use native text selection in thread messages

[saphid]

Problem

Long-pressing message text in the native SwiftUI thread view opens a custom whole-message context menu and highlights the full message surface. Choosing Select text only toggles a hidden selection mode; a second long press still opens the same custom menu instead of immediately presenting native word selection and draggable handles.

Expected

Message text should use normal SwiftUI text selection: long-press the text, select a word or range, and adjust it with native selection handles. Whole-message copy should remain available through the native selection menu and VoiceOver action.

Reproduction

Reproduced on upstream/t3code/rebuild-mobile-app-swift at f98cab553 with a one-paragraph user message in the iOS Simulator.

Workflow

Implementation is being prepared on saphid/swiftui-text-selection; physical-device validation will be performed against personal/swiftui-approved before upstream merge preparation.

[saphid]

Implemented and preserved on upstream-ready branch saphid/swiftui-text-selection at 8d5695446d41. Focused simulator verification: 24 tests passed, 0 failed; native prose and fenced-code selection handles verified, with the standard edit menu plus Copy message. Final direct Claude Opus 5 high review exited 0 with no actionable findings. Disposable validation commit 0b8e253ef rebased as one commit atop personal/swiftui-approved; merged approved-branch verification passed 31 tests, 0 failed. Installed and launched personal Debug build 31 on Alex’s iPhone for explicit approval testing. Upstream preparation is intentionally paused pending that approval.