namir-ui: silent meters render as "-inf dBFS"

[ErwanLegrand]

crates/namir-ui/src/meter.rs:192, 196

format!("{:.1} dBFS", reading.peak_db) renders MeterReading::SILENT's f32::NEG_INFINITY as -inf dBFS in both the bar text and the hover text -- the label shown before any audio arrives.

Fix: render as <= -60.0 dBFS (or an em dash) at or below METER_FLOOR_DB.

[ErwanLegrand]

Fixed in #145 (63afed5). format_db (crates/namir-ui/src/meter.rs:51-56) renders ≤ -60.0 dBFS (a real U+2264, METER_FLOOR_DB) at or below the floor, and for NaN, matching what normalize_db already degraded those to; above the floor it is unchanged. Both call sites go through it — the bar text (meter.rs:65) and the hover text (:70-71) — so neither can print -inf again.

Covered at meter.rs:106-144: one test asserts through the real widget that MeterReading::SILENT paints the bound, and the unit tests pin format_db(f32::NEG_INFINITY) == format_db(METER_FLOOR_DB) == "≤ -60.0 dBFS" while -12.34 still reads -12.3 dBFS.

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