fix(tui): clear selection on right-click copy + group transcript blocks

Two TUI polish fixes.

(1) Right-click copy now clears the highlight.
The right-click handler copied an active selection via onCopySelectionNoClear
(the copy-on-select variant that keeps the highlight during a drag) and never
cleared it, so after right-click-to-copy the selection stayed lit with no
confirmation and a follow-up right-click re-copied the stale range instead of
pasting. A successful right-click copy now clears the selection and notifies;
if the copy fails (no clipboard path) the highlight survives and we fall back
to the right-click paste handler, exactly as before.

(2) Group transcript blocks so boundaries read clearly.
Model replies, reasoning/tool trails, and system/error notes rendered with no
vertical separation, so distinct block types butted together and were hard to
scan. Group adjacent blocks by kind: one blank line opens only where the visual
group changes (model prose <-> reasoning/tool trails <-> notes), while a run of
same-kind blocks renders flush. The rule lives in domain/blockLayout.ts
(messageGroup + hasLeadGap) and is applied intrinsically in MessageLine via a
`prev` prop, which fixes the things ad-hoc per-block margins kept breaking:

  - Streaming stability: the gap is derived from the stable predecessor, never
    the live block's own changing text, so the actively-streaming reply computes
    the same gap while it streams as the settled segment does once it flushes.
    No reflow/jump.
  - Transparent empty trails: a trail hidden by /details, or one carrying only a
    token tally (the finalDetails segment message.complete appends), renders
    nothing and is transparent to grouping (prevRenderedMsg skips it), so there
    are no floating gaps, no doubled gap after a prompt, and no padded space
    above the final reply. In the default/collapsed modes content-bearing trails
    always render, so the grouping is a no-op there.

The virtual-height estimator counts the group-boundary line so scroll math
stays accurate before Yoga remeasures.

ui-tui/src/domain/blockLayout.ts (new), components/messageLine.tsx,
components/streamingAssistant.tsx, components/appLayout.tsx,
lib/virtualHeights.ts, app/useMainApp.ts.

Tests: blockLayout.test.ts (grouping + hidden/empty-trail visibility),
virtualHeights leadGap, app-mouse.test.ts copy behavior. Full ui-tui suite
green apart from 3 pre-existing local/env failures (cursorDrift, ink-resize,
virtualHeights user-prompt-width) unchanged from main.

ui-tui/src/components/appLayout.tsx

@@ -8,6 +8,7 @@ import { $isBlocked, $overlayState, patchOverlayState } from '../app/overlayStor
 import { $uiState } from '../app/uiStore.js'
 import { INLINE_MODE, SHOW_FPS, TERMUX_TUI_MODE } from '../config/env.js'
 import { PLACEHOLDER } from '../content/placeholders.js'
+import { prevRenderedMsg } from '../domain/blockLayout.js'
 import {
   COMPOSER_PROMPT_GAP_WIDTH,
   composerPromptWidth,
@@ -125,6 +126,11 @@ const TranscriptPane = memo(function TranscriptPane({
                   detailsMode={ui.detailsMode}
                   detailsModeCommandOverride={ui.detailsModeCommandOverride}
                   msg={row.msg}
+                  prev={prevRenderedMsg(
+                    i => transcript.virtualRows[i]?.msg,
+                    row.index,
+                    { commandOverride: ui.detailsModeCommandOverride, detailsMode: ui.detailsMode, sections: ui.sections }
+                  )}
                   sections={ui.sections}
                   t={ui.theme}
                 />
@@ -141,6 +147,7 @@ const TranscriptPane = memo(function TranscriptPane({
             compact={ui.compact}
             detailsMode={ui.detailsMode}
             detailsModeCommandOverride={ui.detailsModeCommandOverride}
+            prevMsg={transcript.historyItems[transcript.historyItems.length - 1]}
             progress={progress}
             sections={ui.sections}
           />