feat(notice): show full toast text by default, top-right position, hover-pause dismiss

[northwang-lucky]

Problem

Extension notifications (the floating NoticeShelf toasts) currently:

1. Truncate long messages — the single-line nowrap + ellipsis span inside a fixed 60px / max-620px container gives users no way to read the full text (e.g. pi-fff's multi-field health report).
2. Swallow embedded newlines — senders like pi-fff compose multi-line messages with lines.join("\n"), but white-space: nowrap/normal folds every \n into a space, so structured output renders as one run-on line.
3. Auto-dismiss after 5s with no way to pause, so a toast can disappear before the user finishes reading.

Changes

Toasts show their full text by default (components/ChatWindow.tsx, NoticeShelf):

• Message span uses white-space: pre-line + word-break: break-word: embedded \n are preserved and long lines wrap within the existing max-width: min(100%, 620px) instead of truncating. max-height: 500 remains as a safety cap for pathological messages.
• Toasts move from top-center to the top-right corner (justify-content: flex-end on the wrapper, align-items: flex-end on the shelf column), so each toast's right edge stays pinned and its width extends leftward as content grows.
• transformOrigin follows the new anchor (top right).
• The type indicator dot is top-aligned with the first text line on multi-line toasts (instead of vertically centered), and the font size is reduced from 18 to 14 for a more compact look.
• Content taller than the 500px cap scrolls inside the text area (thin scrollbar); the type dot stays pinned at the top and does not scroll.

Hover pauses the dismiss timer (hooks/useAgentSession.ts):

• The hook exposes setNoticeHover(id); while any notice is hovered, no expiry timer runs. Elapsed time is accrued via remainingMs/startedAt refs, so the countdown resumes from the remaining time instead of restarting at 5s. The timer resets whenever the oldest visible notice changes.

Two latent blockers fixed along the way:

• The floating wrapper is intentionally click-through (pointer-events: "none"), which meant hover events never reached the toast itself; each item now opts back in with pointer-events: "auto" (the rest of the overlay stays click-through).
• The notice-shelf-in entrance animation ran with fill: both and its to keyframe pins height/min-height/max-height: 60px — a completed forwards-fill animation keeps winning over inline styles and would have clamped expanded toasts forever. Fill is now backwards (entrance visuals unchanged).

Note: an earlier iteration of this PR implemented hover-to-expand instead. It was dropped because expanding changed the toast's width (fit-content shrinks once text wraps), which could eject the cursor and cause an enter/leave flicker loop. Always-expanded has a stable hover area and is simpler; hover now only controls the dismiss timer.

Behavior

	before	after
long text	truncated with …	full text always visible
embedded \n	folded into spaces	preserved (pre-line)
position	top-center	top-right, right edge pinned
5s auto-dismiss	always	paused while hovered, resumes on leave

Testing

• tsc --noEmit clean, npm run lint clean.
• Verified in a real browser (Chrome via CDP against next dev) with pi-fff's 7-line health notice: toast renders fully expanded at the top-right by default, survives well past 5s while hovered, and the countdown resumes and completes after the mouse leaves.

Known accepted edge: when more than MAX_NOTICES are queued, an incoming notice can still force the oldest one out even while hovered (pre-existing queue-overflow behavior, unchanged).