Add F1-F5 as default tab-switching shortcuts

[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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With 5 possible tabs (Playback/Recording/Output/Input/Configuration), cycling one at a time with H/L or Tab/Shift+Tab is slow if you want to jump straight to a specific one.

This has been asked for before - #38, closed by the reporter themselves once they found the workaround, not because it was rejected. SelectTab(N) already exists and already works exactly for this, but has no default binding, so users have to know to add it themselves via the README's own "Use F-keys to select tabs" customization example.

Makes that the default instead:

• F1-F5 bound to SelectTab(0..4).
• SelectTab already silently no-ops on an out-of-range index (see Action::handle), so binding all 5 unconditionally is safe even with tabs configured to fewer than 5 entries - unused F-keys just do nothing rather than erroring.
• Updated the README's keybinding table and its now-redundant "F-keys to select tabs" example, rewritten to show remapping F-keys to something else (e.g. Alt+1..5) for anyone whose terminal/WM intercepts function keys, since the original example no longer demonstrates anything you don't already get by default.

New default keybindings:

keybindings = [
 # Select a tab by position (Playback/Recording/Output/Input/Configuration,
 # or whatever order/subset `tabs` is set to)
 { key = { F = 1 }, action = { SelectTab = 0 } },
 { key = { F = 2 }, action = { SelectTab = 1 } },
 { key = { F = 3 }, action = { SelectTab = 2 } },
 { key = { F = 4 }, action = { SelectTab = 3 } },
 { key = { F = 5 }, action = { SelectTab = 4 } },
]

Like every other default binding, these are fully overridable/removable from the config file.

Tested:

• cargo test: 145/145 passing, including a new test asserting all 5 F-key defaults resolve to the expected SelectTab index
• cargo fmt --check / cargo clippy -- -D warnings / cargo doc (matching this repo's CI): all clean

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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.