ui: drop the footer cheatsheet; F1 opens a key reference instead

The permanent Footer spent a screen row on a truncated, always-visible key list.
Remove it (the status line now owns the bottom row) and put the full cheatsheet
behind F1 — grouped by task (Navigate / Views / Move / Edit / Search) rather than
by widget, which is what makes it readable. Esc, F1 or q closes it; there's also
a "Keyboard shortcuts" command-palette entry.

F1 rather than '?' because '?' is already search-backwards in the code views.
Textual leaves F1 unbound (App only claims ctrl+q/ctrl+c), so it traps cleanly.

Gotcha worth recording: the helper that builds the cheatsheet was first called
_render, which collides with Widget._render — Textual invoked ours internally and
got a rich Text where it wanted a Visual, so the whole screen failed to paint
("'Text' object has no attribute 'render_strips'"). Renamed to _cheatsheet.

The search scenario's "rendered above the footer" check now asserts the search
input owns the bottom row instead. New `help` scenario: footer gone, F1 opens it,
groups + real bindings present, Esc closes (5 checks).

Suite note: view_toggle fails in a full run ONLY as collateral from the standing
`filter` flake, which runs immediately before it — when filter leaves the table
empty, view_toggle's open_biggest has nothing to open. Verified: view_toggle
passes alone (13/13) and directly after `help` (18/18), and reproduces as a
cascade with --only filter,view_toggle. Not a regression from this change.

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