Add lenient_config to ignore unknown config fields instead of failing

[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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Every config struct currently uses #[serde(deny_unknown_fields)], so any unrecognized field anywhere in wiremix.toml - a typo, or a key from a different wiremix version than the config was written for - makes wiremix refuse to start at all, with no way to opt out. This came up concretely on my own machine: I keep a couple of not-yet-upstream fork-only theme keys (row_selected, row_unselected) in my config, and anything invoking stock /usr/bin/wiremix directly (bypassing PATH) would fail outright rather than just not styling those rows.

This adds a lenient_config config option (default false, so today's strict-by-default behavior is unchanged) with matching --lenient-config/--no-lenient-config CLI flags, following the same pattern as mouse/--mouse/--no-mouse and friends. When enabled, unknown fields are logged as warnings and ignored instead of erroring:

$ wiremix --lenient-config
wiremix: warning: ignoring unknown configuration field 'totally_unknown_field'

• Uses the serde_ignored crate to collect every ignored field's path across the whole file in one pass - not just the first one hit - regardless of nesting depth (themes.default.foo, [[keybindings]] entries, etc.), so both the default strict error and --lenient-config's warnings report everything at once rather than one-at-a-time-per-run.
• Required removing #[serde(deny_unknown_fields)] from the individual structs (ConfigFile, Keybinding, Names, Filter, ThemeOverlay, StyleDef, CharSetOverlay, and the raw struct behind NameOverride's custom Deserialize impl) - serde_ignored's callback only fires for fields a struct's own Deserialize impl would otherwise silently drop, and deny_unknown_fields makes that struct hard error before serde_ignored ever gets a chance to see it, bypassing the lenient/strict decision entirely.
• The one exception is the test-only strict::ConfigFile used to validate that the shipped example wiremix.toml documents every field exhaustively - that one keeps deny_unknown_fields, since catching drift there is its whole purpose and has nothing to do with runtime behavior.

Tested:

• cargo test --release: 147/147 passing (includes new tests for: unknown fields at the top level, inside [[keybindings]], [names]/[[names.overrides]], [themes.*], and [char_sets.*] all still erroring by default; lenient_config = true in the file and --lenient-config on the CLI both suppressing the error; --no-lenient-config overriding a lenient_config = true set in the file back to strict)
• cargo fmt --check / cargo clippy --release --all-targets -- -D warnings / cargo doc --no-deps (matching this repo's CI): all clean
• Live-verified with a real config file containing an unrecognized field: fails to start with a clear error by default; starts normally and prints a warning to stderr with --lenient-config; a lenient_config = true line in the config file has the same effect, and --no-lenient-config on the command line correctly overrides that back to strict