Adds a single window-level key dispatcher in the root layout: `g` then
c/t/f/p/s (or the digits 1–5) jump between the five sections, honouring
each section's remembered URL so you land back on the same filter and
scroll. `?` toggles a shortcuts cheat-sheet overlay. The handler stays
out of the way while typing in inputs or when a browser/OS modifier is
held. This is the foundation for the per-context keymaps (grid, viewer,
tag/filter pickers) that follow.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
