feat(desktop): follow streaming output at bottom + jump-to-bottom button (#45263)

Strict sticky-bottom autoscroll for the chat thread: while the viewport is
parked at the bottom, the tail follows content growth (streaming tokens, late
measurement, Shiki re-highlight) via a useLayoutEffect keyed on the
virtualizer's own size signal, pinned in the same pre-paint pass as its
scrollToFn so the two never rubber-band. The gate is a single boolean — one
upward pixel (scroll/wheel/touch) disarms follow until the user returns to the
bottom.

Adds a floating jump-to-bottom control that appears once scrolled ~10px away
(above the dim threshold so a sub-pixel settle never flashes it), positioned
above the composer with respect to the status stack, with a subtle
scale + slide in/out animation that honours prefers-reduced-motion. The button
bridges to the virtualizer's re-arm + pin path through a small nanostore
emitter.

Supersedes #43624.

apps/desktop/src/i18n/en.ts

@@ -1666,6 +1666,7 @@ export const en: Translations = {
       stopReading: 'Stop reading',
       readAloud: 'Read aloud',
       editMessage: 'Edit message',
+      scrollToBottom: 'Scroll to bottom',
       stop: 'Stop',
       restorePrevious: 'Restore previous checkpoint',
       restoreCheckpoint: 'Restore checkpoint',

apps/desktop/src/i18n/types.ts

@@ -1325,6 +1325,7 @@ export interface Translations {
       stopReading: string
       readAloud: string
       editMessage: string
+      scrollToBottom: string
       stop: string
       restorePrevious: string
       restoreCheckpoint: string

apps/desktop/src/i18n/zh.ts

@@ -1846,6 +1846,7 @@ export const zh: Translations = {
       stopReading: '停止朗读',
       readAloud: '朗读',
       editMessage: '编辑消息',
+      scrollToBottom: '滚动到底部',
       stop: '停止',
       restorePrevious: '恢复上一个检查点',
       restoreCheckpoint: '恢复检查点',