feat: move header details into ETF count popover

[daggerok]

Shared header behavior

• Keep only N selected: TICKER1, TICKER2 next to the brand title; empty when none selected. Sorted, clickable tickers keep active highlighting, including when all are selected.
• Move the existing rich summary into the count badge popover: instructions/context, update time, counts, API/source links and fallback attribution. Original DOM nodes/listeners are retained, not flattened into a native title.
• Mouse hover, focus, touch/click, Escape/outside dismissal, hover-to-panel and light/dark viewport-safe positioning.
• Existing renderer/selection/data/updater/export/table behavior retained. No dependency or financial data changes.

Validation

Four new tests of the actual header helper. Bun suite: 50 pass, 0 fail. Header browser harness passed zero/two/all/clear, activation, contextual details, hover-to-panel, keyboard/Escape, touch, dark/mobile and inline upload mode where applicable.
Full app browser checks passed with the real static catalog and real checkbox selection, ticker activation/highlight, Clear, dark/mobile and no page errors. Tests are browser/runtime checks, not a claim of semantic tsc validation.

Base: feat/aberdeen-plan-recovery, confirmed by Pages API as the published source branch. Original implementation PR #1 remains open and is NOT merged. Owner removed the standalone UI manifest in 283109c; the parity test now reads its existing canonical .worklog.txt archive. Exact new substitutions were checkpointed there before this PR; reversed pinned reference hash still matches. Deleted files are not recreated.

The user explicitly authorized creating and merging this shared-header UI change across all ETF repos without further approval. Scope and outcomes are tracked in Aberdeen .worklog.txt and .plans/header-rollout.json.