Page through the TUI's lists with PgUp and PgDn

[dhh]

PgUp and PgDn did nothing in the TUI's lists; only the journal handled them. A long Imbox could only be moved through one thread at a time.

• The change: PgDn scrolls a list on by the window it shows, like a pager: the threads just past the bottom come up to the top, and the cursor keeps its row on screen. PgUp goes back the same way. PgDn also reads the next page in, as the down arrow does near the bottom.
• At the ends: on the last window, PgDn moves the cursor to the last thread, and PgDn never scrolls the last thread off the bottom (lastWindowStart). On the first window, PgUp moves the cursor to the first thread.
• Where: every mail list (boxes, labels, collections, search results, bundles, Previously Seen), both panes of The Screener, and the contact list.
• How: posting rows vary in height because of the section headers, so contentList gets its own pageDown/pageUp built on visibleItemsFrom. The Screener and contacts have fixed-height rows and share a small pageBy helper.

docs/tui.md mentions the keys. New tests cover paging a posting list both ways, stopping at the ends, a list shorter than the window, pageBy's cases, and the keys reaching the list through the mail view.

🤖 Generated with Claude Code

[Copilot]

Copilot review overview

🟡 Changes recommended

Section headers currently cause the cursor to shift screen rows, contrary to the documented paging behavior.

Review effort: Balanced
Findings: 1 · 1

Open (2)

• Preserve rendered-line offset when paging across section headers · New
• Limit paging claim to supported mail, Screener, and contact lists · New

What changed in this PR

Adds PgUp/PgDn navigation to paginated TUI lists.

Changes:

• Implements page-wise navigation for mail, Screener, and contact lists.
• Preserves pagination loading behavior.
• Adds tests and TUI documentation.

[!TIP]
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File	Description
internal/​tui/​content.go	Adds paging algorithms.
internal/​tui/​mail.go	Routes page keys across mail lists.
internal/​tui/​screener.go	Adds Screener paging.
internal/​tui/​contacts.go	Routes contact-list page keys.
internal/​tui/​contacts_list.go	Adds contact paging methods.
internal/​tui/​tui_test.go	Tests paging behavior and routing.
docs/​tui.md	Documents page keys.

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