feat: unified @mention button in FileViewer (lines or whole file)

[windli2018]

What

The FileViewer (opened-file tab) tool bar now has a single @ mention button that works everywhere:

• Selected lines in source mode → inserts a line-range mention @path#Lstart-Lend
• Otherwise → inserts a whole-file mention @path

The button is always visible (previously the line-mention button only showed in source mode). Behavior follows the current selection — same @ icon, one entry point. A dynamic tooltip shows what will happen: "Mention selected lines (L12-L18)" vs "Insert path".

On mobile, after any mention the file panel/sidebar closes so the chat (with the inserted mention) is revealed — no need to manually navigate back.

Why

• File mention (@path) was missing in FileViewer entirely (only the file explorer sidebar had it).
• The existing line-mention button was source-mode-only and disabled without a selection, so it was hard to discover.
• One button that adapts to context keeps the toolbar clean while covering both cases.

How

• FileViewer: a unified mention button (onPointerDown preventDefault to preserve the selection through the click on touch devices). Click handler branches on selectedLineRange.
• AppShell: pass onAtMention to FileViewer (reusing the existing handleAtMention); the three mention handlers now collapse the file panel + sidebar on mobile so the user lands back in the chat.

Notes

• Line mention still requires onMentionLines (passed when the right panel is open); when it's absent the button falls back to whole-file mention — so the button is always useful.
• No new i18n keys (reuses existing `files.insertPath` / `files.mention` / `i18n.mentionSelectedLines`).