web-ui: selection actions toolbar

[time-attack]

Summary

Selecting text inside a message shows a floating pill toolbar under the
selection: Quote, Explain and Ask insert the quoted passage (and the typed
question) into the composer and focus it — never auto-submitting — and Copy
copies it. The toolbar survives Escape and the transcript's auto-scroll, resolves
read-only views from state, and the message stack's selection highlight is the
blue tint.

Demo

Selecting a sentence in a reply: the floating toolbar appears; Quote drops the quoted passage into the composer and focuses it.

Verification

Typecheck, the full plugins/web-ui suite, eslint and prettier pass at this level of the stack (each level is verified on its own, on top of the previous one). Live QA of the finished stack ran in a browser-only dev-instance against a seeded conversation (tool run, markdown table, TypeScript and diff fences, a pending approval, ⌘K, list pages, settings), in light and dark and at phone width.

Stack

Merge in order; each PR is based on the one above it and retargets automatically when its base merges.

#	PR	Change
1	#1053	Beautiful UI foundation — tokens, fonts, styles scaffold
2	#1054	loading state and the live thinking trace
3	#1055	chat rows, session header and empty state
4	#1056	approval card with a pager
5	#1057	tool chips and code blocks
6	#1058	records tables and filter lists
7	#1059	search palette
8	#1060	settings as inspector cards
9	#1061 (this)	selection actions toolbar
10	#1062	recommendation card

[time-attack]

Closing per review: holding off on importing Beautiful UI wholesale for now (interactions read as too stimulating and too Conductor-like). A subset may come back as our own take in collaboration with Eve. Branch bui/09-selection-actions is kept.