Add PageUp/PageDown/Home/End navigation

[HoneyHazard]

⚠️ Full Disclosure: Drafted with AI assistance (Claude); reviewed by me briefly. I am neither a RUST developer nor pipewire expert. If I should stop making these PRs into your wonderful project, please let me know. ⚠️

That being said, I hope these can be helpful and useful additions that users could appreciate.

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wiremix had no way to move the selection by more than one item at a time - only MoveUp/MoveDown (j/k, arrows), which is slow to navigate with on a system with many devices/streams. PageUp/PageDown/Home/End are all listed as valid SpecialKey values in the keybinding docs, but no actions existed that did anything more than single-item movement, so binding them to the existing MoveUp/MoveDown actions wouldn't have given real paging/jump behavior.

Adds four new actions, bound by default to the matching keys:

• PageUp/PageDown jump the selection by however many items are currently visible on screen (ObjectList already computes this every frame for scroll positioning - now cached as page_size and reused for paging), clamping to the first/last item if the page would overrun the list.
• MoveFirst/MoveLast (Home/End) jump straight to the first/last item, reading directly from View::object_ids() rather than walking one item at a time.
• All four are also wired into the help menu's scroll (a fixed 10-line step for PageUp/PageDown there, and 0/u16::MAX for Home/End relying on HelpWidget::render()'s existing bottom-of-list clamp - the help view doesn't track a visible-line count the way the main object lists do).

New default keybindings:

keybindings = [
 # Select the item a page down (however many items are visible at once)
 { key = "PageDown", action = "PageDown" },
 # Select the item a page up
 { key = "PageUp", action = "PageUp" },
 # Select the last item
 { key = "End", action = "MoveLast" },
 # Select the first item
 { key = "Home", action = "MoveFirst" },
]

Like every other default binding, these are fully overridable/removable from the config file (e.g. { key = "PageDown", action = "Nothing" } to disable).

Tested:

• cargo test: 150/150 passing, including 6 new tests covering page movement, first/last jumps, overflow-clamping at both ends, and the help-menu case for all four actions
• cargo fmt --check / cargo clippy -- -D warnings / cargo doc (matching this repo's CI): all clean
• Manual verification in a live terminal at two different sizes - one where paging overflows to the list's end, one where it's a real mid-list jump