`:` opens the command palette, and a test proves the `?` help lists every key the page answers #6748

Keyboard-first like k9s and lazygit (#3697, folded into the direction). src/web/client.ts answers j, k, o/Enter, c, /, ? and Cmd/Ctrl K in its document keydown handler, and STATIC_KEYS feeds the help overlay; nothing checks that the two agree, and : does nothing.

Done when:

• [ ] Outside a field, : opens the palette (openPalette()), as Cmd/Ctrl K does; inside a field it types a colon.
• [ ] STATIC_KEYS lists : with "Command palette".
• [ ] A new test reads src/web/client.ts and fails when a key the handler answers (e.key === '…') has no row in STATIC_KEYS, or a row names a key the handler never answers.

Files: src/web/client.ts, one new test file.