feat: scroll the message list with PageUp/PageDown

[LawrenceGK]

Description

Adds keyboard paging over the conversation view via PageUp / PageDown.

On desktop the input box is usually focused, so the browser never scrolls the (react-virtuoso) message container on its own when these keys are pressed. This wires them up to scroll the conversation explicitly, driving the VirtuosoHandle already exposed on uiStore (messageScrolling). Each press scrolls by ~90% of the viewport height as an instant jump, matching native PageUp/PageDown. The keys don't conflict with text input, so they work regardless of focus.

The shortcut is also surfaced in Settings → Hotkeys as two read-only rows ("Scroll Up / Down the Message List"), kept hardcoded like the other non-configurable shortcuts (e.g. mod+1-9). Added Page Up / Page Down key labels to formatKey, plus en / zh-Hans / zh-Hant translations.

Files changed:

• src/renderer/hooks/useShortcut.tsx — PageUp/PageDown handler driving the VirtuosoHandle
• src/renderer/components/Shortcut.tsx — two read-only Hotkeys rows + Page Up / Page Down labels
• src/renderer/i18n/locales/{en,zh-Hans,zh-Hant}/translation.json — new strings

Additional Notes

• No new settings field — the shortcut is hardcoded like the existing ones handled in useShortcut, so it isn't user-rebindable.
• Self-contained: touches only the renderer shortcut handler, the Hotkeys settings list, and translations.
• Other locales fall back to English (fallbackLng: 'en') and can be filled in later by the existing translation pipeline.

Screenshots

N/A — keyboard-only interaction. The only visual change is two read-only rows added to Settings → Hotkeys.

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[themez]

Thanks for the PR. I checked the diff and tried applying it locally. The patch is mergeable and VirtuosoHandle.scrollBy() is available in the current react-virtuoso, but I do not think this should be merged as-is.

Main blocker: the new PageUp/PageDown handler is registered on global window.keydown. As long as messageScrolling exists, it calls preventDefault() and scrolls the message list regardless of where focus currently is. That means PageUp/PageDown inside Settings, Search, other dialogs, inputs/textareas, or contenteditable areas can be intercepted and scroll the chat behind the active UI. The PR description says these keys do not conflict with text input, but in practice they are native paging/navigation keys in many focused controls and modal surfaces.

Suggested fix: only handle PageUp/PageDown when the active context is the chat page/message input/body, and explicitly skip dialogs/modals plus input, textarea, select, and [contenteditable] targets.

One more project-specific issue: this PR hand-edits src/renderer/i18n/locales/*/translation.json. These locale files are generated in this repo, so new i18n keys should be added from source and translations should go through the existing translation generation flow rather than editing generated locale JSON directly.