What changed

Signal strength no longer shows as a percentage next to the device name — it's now a small bar indicator at the end of the device title, similar to a cellular signal icon. This fixes a long-standing confusion where users would read "93%" as a battery level since both numbers looked identical.

The key and direct-connection icons that used to be bundled with the signal percentage now live on their own on the device type line, in a smaller badge.

Technical Context

• New SignalIndicator composable draws four rounded bars via Canvas (no dependency on the Material icon library, which only ships SignalCellular0Bar and SignalCellular4Bar in twotone — missing the 1/2/3 variants).
• Bar thresholds: >=80% → 4 bars, >=60% → 3, >=40% → 2, >=20% → 1, below → 0. Not-live devices render a LinkOff icon.
• Signal quality mapping is extracted into a pure signalLevelOf helper and unit-tested for boundaries, non-finite input (NaN, infinities), and out-of-range values — PodDevice.signalQuality can emit negatives for extreme RSSI, which the mapper now clamps defensively.
• SignalBadge was renamed to DeviceConnectionBadge and stripped of the antenna icon, signal text, and LinkOff branch — it now only carries the BLE key state + AAP direct-connection icons. It early-returns when both are absent, so devices with neither no longer render an empty pill.
• The popup and unknown-device card intentionally do not get the DeviceConnectionBadge — those surfaces never showed key/AAP info, and adding it is out of scope.
• Accessibility: new signal_indicator_bars_cd / signal_indicator_disconnected_cd strings, applied via semantics { contentDescription = ... } on the indicator root. English-only for now — non-English locales will be filled in by a follow-up Crowdin run.
• Dead code removed: PodDevice.getSignalQuality and the already-dead BlePodSnapshot.getSignalQuality.
• Play Store screenshot baselines render DualPodsCard / SinglePodsCard / CasePopUpContent, so they'll drift after this change — intentionally not regenerated here; will be handled in a separate screenshot refresh.

Closes #463