tui: improve testability and simplify code

[dgageot]

Summary

This PR improves the TUI's testability and simplifies its code without removing any feature.

Testability

• New pkg/tui/internal/editorname package: hosts FromEnv(visual, editorEnv) string (previously duplicated verbatim in pkg/tui/tui.go and pkg/tui/page/chat/chat.go — the chat copy was dead code with the only test). Added a regression test for non-ASCII editor names.
• New Transcriber interface in pkg/tui with a WithTranscriber option, decoupling appModel from the concrete *transcribe.Transcriber. Tests can now drive the speech handlers via a fake without audio hardware or network.
• New tests for previously-untested helpers:
  • parseCtrlNumberKey
  • mouse hit-testing (hitTestLeanRegion, hitTestFullRegion — extracted as pure free functions)
  • terminal window title (formatWindowTitle — extracted as a pure helper)
  • speech-to-text handlers (handleStartSpeak, handleStopSpeak)

Simplification

• New pkg/tui/dispatch.go: centralises the Update → type-assert → reassign boilerplate behind updateChatCmd/updateEditorCmd/updateDialogCmd/updateCompletionsCmd and the matching forwardX shortcuts. ~30 repetitions of the 5-line pattern in tui.go/handlers.go collapse to one-liners.
• New pkg/tui/tabs.go: tab persistence and lookup logic (persistActiveTab, persistFreshTab, restoreTabs, persistedSessionID, findTabByPersistedID) lifted out of tui.go.
• copyToClipboard(text, successMsg) helper deduplicates the two clipboard handlers.
• isErrTitleGenerating (string-comparison) replaced with errors.Is(err, app.ErrTitleGenerating).
• persistSplitDiffView(enabled bool) replaces an anonymous goroutine in handleToggleSplitDiff.
• applyThemeChanged shrinks from 12 lines of boilerplate to a 3-line tea.Batch.

Bug fixes (found during self-review)

• UTF-8 corruption in editorname.FromEnv (carried over from the original duplicated copy): the fallback capitalisation strings.ToUpper(baseName[:1]) + baseName[1:] slices by byte, not rune, mangling editor names that begin with multi-byte UTF-8 characters (e.g. édit → garbled output). Now uses utf8.DecodeRuneInString + unicode.ToUpper. Regression test added.
• Data race in tui_exit_test.go (pre-existing, exposed by go test -race): neutralizeExitFunc mutated the package-level exitFunc while the safety-net goroutine spawned by cleanupAll still read from it, and two parallel tests both mutated the global. Fix: shrink shutdownTimeout in tests, wait for the safety-net goroutine before restoring originals, and remove t.Parallel() from tests that mutate package globals. go test -race ./pkg/tui/... now passes for the whole subtree.

Stats

• pkg/tui/tui.go: 2544 → 2301 lines (-243).
• pkg/tui/handlers.go: 674 → 660 lines.
• New: pkg/tui/dispatch.go (82 lines), pkg/tui/tabs.go (171 lines), pkg/tui/internal/editorname/ (76 + 130 lines test).
• New tests: pkg/tui/tui_helpers_test.go (243 lines), pkg/tui/tui_speak_test.go (~130 lines).

Validation

• go build ./... ✅
• go vet ./pkg/tui/... ✅
• go test ./pkg/tui/... ✅
• go test -race ./pkg/tui/... ✅
• golangci-lint run ./pkg/tui/... ✅ (0 issues)

Assisted-By: docker-agent