Add show_dividers option to draw a line between items

[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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Item rows already have a gap between them (NodeWidget::spacing()/DeviceWidget::spacing()), but that space is always blank. On a list with many similarly-styled rows it can be hard to tell at a glance where one item ends and the next begins, especially on a low-contrast theme.

Default theme with --show-dividers on the command line.

Adds show_dividers: bool (default false) plus a new divider theme key and divider char_set glyph:

• When enabled, draws a one-row line spanning the item's width, styled by theme.divider and built from char_set.divider (a single glyph, repeated to fill the width - same pattern as volume_filled/volume_empty).
• The divider is centered in the gap between items - one blank row above it, one below - rather than hugging whichever item happens to sit above it. To make room for that, turning show_dividers on reserves one extra row of spacing per item (so fewer items fit on screen at once than with it off); with show_dividers off, spacing - and the whole list's layout - is completely untouched, matching stock wiremix exactly.
• No divider is drawn after the last visible item, so it never touches the footer/tab bar.
• Off by default, so it's a no-op for any existing config.
• Both the Playback/Recording/Output/Input Devices node list (NodeWidget-based) and the Configuration tab's device list (DeviceWidget-based) go through the same object_list.rs render loop, so a single render_divider() helper covers both list kinds.

New config keys (all optional, no-op when unset):

show_dividers = false

[themes.default]
divider = { fg = "DarkGray" }

[char_sets.default]
divider = "─"

Also available as --show-dividers/--no-show-dividers on the command line.

Tested:

• cargo test --release: 148/148 passing, including tests covering the disabled no-op case, the enabled case drawing the glyph centered with a blank row on each side, clipping at the list's bottom edge without panicking, and that show_dividers reserves the extra row of spacing only when it's actually on (confirmed by comparing how many items fit in an identical Rect with it on vs. off)
• cargo fmt --check / cargo clippy -- -D warnings / cargo doc (matching this repo's CI): all clean
• Manually verified in tmux with --show-dividers: the divider line renders centered between every pair of adjacent items, with a real terminal-row-count difference from leaving it off (5 items fit per screen without dividers vs. 4 with, at the same terminal size) - and that leaving the option unset reproduces stock behavior exactly, byte-for-byte the same layout as before this PR

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Drafted with AI assistance (Claude); reviewed by me before opening. Left as a draft while I finish going through it - not requesting review yet.