refactor(live-input): drop the PCM-only pipewire mode, keep the bridge sink

[mgth]

Stacked on #353 (both touch reconcile_live_input); merge that one first.

Why

The PCM-only pipewire live-input mode published a second sink (omniphony_input_7_1, f32 7.1 at 48 kHz) that only accepted linear PCM. The pipewire_bridge sink has advertised a linear-PCM alternative next to IEC 61937 for a while, so the PCM-only sink no longer has a reason to exist. This keeps pipewire_bridge as the sole PipeWire input; renaming it to plain pipewire is a later step.

Renderer

• InputMode::Live and InputModeConfig::Live are gone. The names pipewire and live stay as deserialization aliases of pipewire_bridge (serde, the OSC input/mode control, and Studio's normalize_input_mode), so a saved config or an older client selecting the removed mode lands on the bridge sink instead of failing to parse — a config parse failure would silently fall back to Config::default(). Serialization still writes pipewire_bridge.
• live_input.rs: the PCM capture loop, its config resolver, PipewirePcmInputConfig and the ActiveCaptureConfig enum are removed; the manager works on PipewireBridgeInputConfig directly. The bridge sink's own PCM fallback (process_pcm, DecodedSource::Live) is untouched.
• sample_format was consumed only by the PCM resolver, so it is removed end to end: InputSampleFormat, the request field and setter, live_input.sample_format in the config, the /omniphony/control/input/live/format OSC control and its format key in /state/input, and the --input-format CLI flag (parsed and never read).
• IdleFeedInputs::input_self_feeding only ever described the PCM sink; removed with its test.
• handler.rs drops the (Live, DecodedSource::Live) acceptance arm; session_run.rs no longer computes the self-feeding flag.

Studio

• The PipeWire option of the input mode selector, the Format select, its listener and Tauri command, and the liveInput.format state are removed. normalize_input_mode maps live/pipewire to pipewire_bridge.
• Rows that only the PCM mode ever showed (Backend, Layout, Channels, Sample rate, Map, LFE) stay in the DOM but are hidden and disabled with a comment: the renderer still accepts those request fields, and which of them return is for the PipeWire-mode rework to decide. Placeholders and defaults now match the bridge sink (omniphony, 2 ch, 192 kHz).
• i18n (8 locales): removed input.mode.pipewire, input.format, help.input.format, input.status.live, input.summary.live, input.summary.liveLayout; reworded help.input.mode and input.infoBody. check-i18n reports full parity.

Verification

• cargo fmt --all --check, cargo check --workspace --all-targets and cargo test --workspace on the renderer; cargo check on src-tauri; node --check on the edited Studio files; npm run i18n:check; npm run lint:dead.
• Not exercised at runtime: switching a config saved with input_mode: pipewire — it now resolves to the bridge sink named by live_input.node, with the channel count the file requests (8 is accepted by the bridge resolver).

Not in this PR

• The rename pipewire_bridge → pipewire.
• Map / LFE / layout request fields, which no code path consumes today; they were not specific to the removed mode.
• mpv's truehd-auto-detect.lua keys on render.input_mode from outside this repo; not checked here.

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