Add row_selected/row_unselected theme keys for row highlighting

[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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Selection state currently only reaches SelectorWidget (the 3-row gutter marker) - node_title, node_target, the volume% label, the "muted" label, and (in the Configuration tab) the device name and profile line are all styled identically whether their row is selected or not, with no way to change that from a theme.

Adds a new theme key, row_selected: Style, with these properties:

• Applied as a single buf.set_style(area, ...) fill over the selected row's whole Rect before any of its spans are drawn. ratatui's Cell::set_style only overwrites fg/bg when the incoming style has Some(...) for that field, so unstyled spans (node_title etc., which default to { }) inherit the fill, while spans that already set their own color (meter_active, volume_filled, ...) still paint over it for their own glyphs.
• Covers blank padding/spacer cells between spans that per-span styling could never reach, so the highlight reads as a clean full-width bar rather than patchy text-only coloring.
• Empty ({ }) in all three built-in themes, so it's a no-op unless a theme explicitly sets it - fully backward compatible.
• Covers both the Playback/Recording/Output/Input Devices node list (node_widget.rs) and the Configuration tab's device list (device_widget.rs), which are separate widget implementations and both needed the same treatment.

Here's a screenshot showing an example of row_selected usage:

Also adds two new config options, row_selected_extend_above and row_selected_extend_below (both default false), which extend the row_selected fill:

• Let the row_selected fill bleed one row above and/or below the row itself, into the gap between items, independently in each direction, so the highlight doesn't cut off abruptly right at the row's own edges.
• The selector marker (the left gutter column) extends to match, using the selector_middle glyph for the extra row - but only when row_selected is actually customized away from its default empty style, since drawing extra marker glyphs is real cell content rather than a style patch and can't rely on being inert by default the way the background fill can.
• Clipped to the list's own area, so the extension can never bleed into a neighboring item or past the list into the footer/tab bar.
• Both default to false, so there's no size or behavior change for any existing config unless explicitly turned on.

Here's a screenshot showing an example of row_selected_extend_above usage:

Here's a screenshot showing an example of row_selected_extend_below usage:

Here's a screenshot showing both together:

Finally, adds a second new theme key, row_unselected: Style, that complements row_selected from the other direction:

• Patched onto the text spans of every row that isn't selected - node_title, node_target, the volume% label, the "muted" label, and (Configuration tab) config_device/config_profile.
• Before this PR, those spans had no selection-awareness at all - they render identically whether their row is selected or not. row_unselected is the only hook that lets them differ by selection state, independently of whatever row_selected does to the selected row. That matters for a "make the selection stand out from both directions" look: dim everything else (e.g. fg = "DarkGray") so contrast comes from receding the unselected rows as well as highlighting the selected one, not just from the selected row being brighter.
• Deliberately per-span rather than a whole-row fill (unlike row_selected), since the goal is a faint text-only tint distinguishing item rows from the blank gaps between them, without a second block-style highlight competing with the selected row's own look.
• Empty ({ }) by default in all three built-in themes, i.e. no change from an item's normal look - fully backward compatible.

Here's a screenshot showing an example of row_unselected usage:

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

[themes.default]
row_selected = { fg = "Black", bg = "Cyan", add_modifier = "BOLD" }
row_unselected = { fg = "DarkGray" }

row_selected_extend_above = false
row_selected_extend_below = false

Tested:

• cargo test --release: 144/144 passing
• cargo fmt --check / cargo clippy -- -D warnings / cargo doc (matching this repo's CI): all clean
• Manual verification via raw ANSI capture and buffer-cell inspection in tmux that: the row_selected fill spans the full row width (including blank spacer lines and trailing padding); row_selected_extend_above/_below independently control whether the fill (and matching marker) extend one row above/below; row_unselected tints only the text spans of unselected rows, leaving blank padding/gaps and the selected row untouched; and meter/volume foreground colors still render correctly on top of both - in both the node list and the Configuration tab
• Verified leaving row_selected, row_unselected, and the two extend options unset reproduces stock behavior exactly