feat(serve): Cmd/Ctrl+K command palette

[efij]

Why

Cmd/Ctrl+K only focused the small header field; there was no popup. On a narrow window that field is hidden, so the shortcut silently switched to More › Search.

What

A command palette over the page:

• Empty: recent searches, the findings to look at first, views, actions.
• Typing: fuzzy match over views, More sections, findings (plain title or rule id) and conversations, 5–7 ms per keystroke on a 353k-event / 2,495-finding case. Matching actions from the evidence (/api/search, events only) arrive a moment later, with a visible "Searching every recorded action…" line meanwhile.
• Keys: ↑ ↓ / Tab move, ↵ opens, Cmd/Ctrl+↵ full search (original files included), Esc clears then closes, / opens it too. The header field and a new phone search button open the same palette.
• Dialog + combobox + listbox semantics with aria-activedescendant; focus never left in the hidden input; secrets masked in every result; dark mode; reduced motion.

Existing shortcuts are unchanged: physical-key matching, j/k in Activity, arrows in Findings, Esc closes the drawer.

Verified

• go test ./... green, new TestCommandPalette, existing TestShortcutsMatchPhysicalKeys still passes.
• Chrome (Playwright) script: Cmd+K opens with focus in the box; typing "cloud" lists findings, evidence and the full-search row; ↓ ↵ opens the finding; Esc closes; / opens; Cmd+↵ runs the full search; fuzzy "overvw" → Overview; header click opens; j still works; phone width shows the search button and it opens. No console errors. The first run caught a focus bug (keys typed into the closed palette), fixed and re-verified.
• Large real case: typing stays at 5–7 ms per keystroke; evidence search replaces the pending line when it returns (2–3 s there).

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