Fix Fragment view leak and edge-to-edge/contrast UI polish issues

[kasnder]

Five small, independent UI-polish fixes, batched into one PR per the triage in OPEN_ISSUES_TRIAGE.md (WI-7, WI-8). Draft PR for maintainer review.

Checklist

• Fixes [LeakScope] 16 Android lifecycle/memory violations detected #586 — TrackersFragment held its SwipeRefreshLayout in a field with no onDestroyView(), leaking the View (and Activity context) on every details-screen recreation. Added onDestroyView() that nulls the field.
• Fixes with new wireguard feature the screen is to long #575 — The WireGuard/VPN screen's RecyclerView had no bottom window-inset handling, so the last row was hidden under the gesture/navigation bar. Added a per-view WindowInsetsCompat listener in VpnFragment (mirrors the pattern already used in ActivityLog/ActivitySettings/DetailsActivity/ActivityMain); did not touch ActivityMain.java.
• Fixes Next button on the onboarding screen is partially under the navigation bar #551 — The onboarding Next/Previous buttons used a flat 16dp bottom margin, so they could be clipped by the nav bar on edge-to-edge displays. Added a bottom-inset-consuming margin listener in ActivityOnboarding.
• Fixes Link to the VPN settings with dark mode have bad contrast and is hard to distinguish #560 — The "Manage WireGuard profiles" footer link used ?attr/colorAccent, which in night mode (#4db6ac) reads as muted/low-contrast against the dark surface. Added a brighter dedicated colorFooterLink for night mode and made the text bold so it clearly reads as a link.
• Fixes Traffic log toggle does not appear as a toggle when turned on #559 — The traffic-log toggle's RocketSwitch.NoIcon style zeroed out trackDecorationTint, removing the Material3 track outline and leaving the inactive track looking like a bare thumb rather than a switch. Restored the outline for that style variant only, leaving the rocket-icon main power switch (RocketSwitch, used by ActivityMain) untouched.

Notes

• Branch is based on current origin/master (includes PR Fix bottom nav bar overlap on edge-to-edge displays #597 inset fixes and PR Fix lock deadlocks, production NPE, connection leaks, and log typo #539 code-review fixes), so the diff here is limited to these five changes.
• Did not touch ActivityMain.java, CountriesFragment.java, or OPEN_ISSUES_TRIAGE.md, per scope.
• Traffic log toggle does not appear as a toggle when turned on #559 (switch styling) could not be visually verified/screenshotted in this environment — a maintainer should confirm the traffic-log toggle now reads clearly as a switch in both light and dark mode before merging.
• Link to the VPN settings with dark mode have bad contrast and is hard to distinguish #560's color choice is also worth a maintainer's eyeball pass in night mode.

Test plan

• ./gradlew :app:compileGithubDebugJavaWithJavac — BUILD SUCCESSFUL (includes the Rust wgbridgeBuild step via preBuild; cargo was available in this environment).
• Manual visual check of Traffic log toggle does not appear as a toggle when turned on #559 (traffic-log switch) and Link to the VPN settings with dark mode have bad contrast and is hard to distinguish #560 (footer link) in light/dark mode on a device or emulator.
• Manual check of with new wireguard feature the screen is to long #575 (VPN screen last row) and Next button on the onboarding screen is partially under the navigation bar #551 (onboarding Next/Previous buttons) on an edge-to-edge (gesture nav) device.

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