feat: add an end-user command palette

[cfstcyr]

Summary

• add a lightweight home page command palette with a visible trigger, Ctrl/Cmd+K shortcut, keyboard navigation, and loading/empty states
• extend /.search to keep owner: lookups working while also returning ranked general link search results for HTML and JSON consumers
• cover the new search behavior and home page surface with Go tests

Testing

• docker build -t golink-test -f- . <<'EOF2'
FROM golang:tip
WORKDIR /src
COPY . .
RUN /usr/local/go/bin/go test ./...
EOF2