fix(tui): free arrow keys for transcript scroll, move recall to ^P/^N

[jkyberneees]

Problem

Navigating the conversation history was practically impossible while the input box holds focus:

• With prompt history present, bare ↑ on an empty input always recalled a previous prompt and never scrolled the transcript — the transcript only got arrow keys in the sliver case "empty input + nothing to recall".
• Mouse wheel scrolling is opt-in (--mouse), so it's dead in a default launch.
• The keys that did work (PgUp/PgDn/^U/^D) were documented nowhere reachable — /help listed only slash commands, and the welcome cheatsheet scrolls away.

Fix

• ↑/↓ at the input's edge lines always scroll the transcript (PgUp/PgDn/^U/^D unchanged).
• Prompt-history recall moves to ^P/^N — the readline-standard binding, so shell muscle memory transfers.
• Discoverability: ^P history · ↑ scroll hint in the input placeholder; a proper keys section in /help (scroll / page / recall / --mouse wheel rows); PgUp more added to the footer's off-bottom badge (PgUp more · G latest · ↕ 42%); README key table updated.

Trade-off

↑-for-history muscle memory breaks; recall is now ^P, exactly like a shell prompt. Scrolling the transcript is the far more frequent intent while the input holds focus, so this is the right default.

Verification

• make fmt / vet / lint (0 issues) / test -race / cover — all green; handleKey, showHelp, footer at 100% coverage.
• Regression test: ↑ scrolls the transcript even with prompt history present (leaves histNav/input untouched); ^P still recalls; existing multi-line cursor-movement and sticky-bottom autoscroll tests unchanged and passing.