feat(log-viewer): redesign the header's problems and notifications surfaces

[lukecotter]

Redesigns the two header controls that looked like siblings but aren't the same kind of thing: Log problems (facts about the user's transaction) on the left, Notifications (facts about our parser) behind the bell on the right.

Header bar

• Log problems is a bare icon button — worst-severity glyph plus a count badge, never tinted: severity is carried by the glyph's shape (so it doesn't depend on colour) and the full breakdown is in title/aria-label. Monochrome chrome, coloured content — colour lives in the panel rows and the timeline.
• Both panels are one native-popover shell (AnchoredPopover), replacing two hand-rolled absolutely-positioned panels. Light-dismiss and Escape come for free. They open at zero with a positive empty state.
• Help & documentation and Report an issue move into a ••• menu.
• The header collapses in a measured, staged ladder — log meta, then the bell, then the Inspector toggle, then log problems — each reappearing inside •••, which never collapses. Stage is a pure function of width, reusing the existing computeVisibleCount.
• Three near-duplicate notification/problem models collapse into one LogIssue.

Issue cards

• A 3px rail in the colour the timeline draws for the same issue, derived from MARKER_COLORS so the canvas and the DOM can't drift.
• Summary and message each clamp to two lines; clicking the message expands it. Stack traces keep their line breaks in the editor font.
• Clicking a card runs its action — unless the click ended a text selection, so a stack trace can still be copied.
• Activation is injected: LogIssue carries an opaque IssueAction supplied by whoever produced the issue, so the list no longer imports from features/call-tree and any surface can reuse it.
• An Unsupported log event name card opens a prefilled GitHub bug report (title/labels matching the bug template). An Invalid log line card stays static — its text can echo customer data. The openUrl webview message is guarded to https: URIs.

Testing

tsc -b, eslint and prettier clean; 89 suites / 1241 tests pass; production build OK. Verified in a dev host against sample-app in light and dark themes: clean log, FATAL ERROR log, a >20MB truncated log and an unsupported-event log; the collapse ladder walked wide to narrow; a second log load carries nothing over.