Add configurable gesture activation zones

[Dragonk]

Depends on #3. Until #3 is merged, GitHub shows the prerequisite accessibility-overlay commits in this PR's diff. After #3 lands, the diff should reduce to the configurable gesture-zone changes only.

Summary

Adds user-configurable activation geometry for the Back and Home/Recents gestures:

• Back activation height — how much of the side edge can start Back (10–100%);
• Home/Recents activation width — the horizontal width of the bottom gesture region (10–100%);
• independent Back edge sensitivity — how far inward from the side edge a touch may start (0.5×–4.0×);
• independent Home/Recents bottom-edge sensitivity — how far above the bottom edge a touch may start (0.5×–4.0×);
• the visible bottom indicator now uses the same resolved width as the real touch zone, so at 100% it spans the full bottom activation region;
• a dedicated Gesture Areas settings screen, parallel to the existing App compatibility screen.

Why

The existing fixed gesture zones can be too small for some devices/users, especially the bottom Home/Recents start region. Different screen sizes, cases, grip styles and navigation setups benefit from different activation depths and coverage. This lets the user tune the hit regions without changing the actual swipe-distance thresholds or navigation actions — the gesture recognizer, hold timing, Recents semantics, replay and tap handling are unchanged.

What changed

Back

• Activation height: 10–100%, 10% increments.
• The Back activation region is anchored upward from the lower usable part of the screen (not vertically centered), so the percentage describes the span measured from the bottom.
• Edge sensitivity: 0.5×–4.0× relative to the original 16 dp hit depth (0.5× → 8 dp, 1× → 16 dp, 4× → 64 dp).
• The same setting is shared by the left and right Back zones.

Home & Recents

• Activation width: 10–100%, centered.
• Bottom edge sensitivity: 0.5×–4.0× relative to the original 12 dp hit depth (0.5× → 6 dp, 1× → 12 dp, 4× → 48 dp).
• Home and Recents continue sharing the same bottom region (Recents is the long action of the bottom gesture).
• The visible bottom indicator uses the same resolved horizontal width as the bottom touch zone, so the bar matches the actual activation region 1:1 (10% → central 10%, 100% → full width). Only horizontal width follows the percentage; sensitivity does not widen the bar.

Geometry

• Production and tests share a single pure geometry resolver (computeGestureZoneLayout(settings, screenGeometry)) — no parallel math.
• The bottom Home/Recents band has deterministic precedence in the lower corners: the side Back zones are offset upward by the effective bottom band height, so a vertical gesture in the bottom band reliably belongs to Home/Recents, and Back works immediately above the band. No ambiguous corner overlap.
• Sensitivity scales only the activation-region depth (the starting hit region); the navigation-bar inset is added afterward and is never multiplied.
• Swipe-distance thresholds (Back 24 dp, Bottom 10 dp) are unchanged.

UI

• The four settings live on a dedicated Gesture areas screen, reached from a compact entry card on the main screen (parallel to the existing App compatibility entry).
• Settings persist through DataStore; changes apply immediately after the slider commits (onValueChangeFinished) — no Save button, no app restart, no Accessibility toggle.
• Sliders use local drag state while dragging and persist the snapped value on release, avoiding dozens of DataStore writes / overlay rebuilds per second.

Validation

./gradlew testDebugUnitTest  -> BUILD SUCCESSFUL
./gradlew lintDebug          -> BUILD SUCCESSFUL
./gradlew assembleDebug      -> BUILD SUCCESSFUL
./gradlew assembleRelease    -> BUILD SUCCESSFUL

26 unit tests pass (0 failures, 0 errors), including regression tests for the configurable geometry, the corner-overlap precedence (using the same production geometry helper), and the indicator-width invariant. Lint: 0 errors, only pre-existing warnings. No instrumentation test was run by the build environment.

Real-device validation

Tested using the release APK on a real Xiaomi/HyperOS Android device. Confirmed:

• configurable Back activation height works;
• configurable bottom activation width works;
• independent Back and bottom sensitivity settings work;
• Back/Home/Recents gesture behavior is working;
• dedicated Gesture Areas settings screen works correctly;
• settings persist and apply correctly;
• 100% bottom activation width produces the intended full-width indicator;
• gestures continue to work inside Android system Settings;
• the previous persistent "displaying over other apps" system notification remains absent.

Test APK

I tested this build on a real device, but additional device coverage would be useful. A prebuilt test APK matching this PR is available from my fork:

https://github.com/Dragonk/Ogesture/releases/tag/gesture-zones-test-2026-08-25

APK asset: https://github.com/Dragonk/Ogesture/releases/download/gesture-zones-test-2026-08-25/ogesture-configurable-gesture-zones-release.apk

This is a test build from my fork, not an official upstream release. Because the test APK is not signed with the upstream production key, users with the official Ogesture build installed may need to uninstall it before installing this APK (INSTALL_FAILED_UPDATE_INCOMPATIBLE otherwise). Users already running a Dragonk test build signed with the same debug certificate can update with adb install -r.

Notes / risks

• This PR is currently stacked on and depends on Use accessibility overlays for gesture zones #3 (the Accessibility Overlay migration). Once Use accessibility overlays for gesture zones #3 merges, this branch can be rebased onto main and the diff reduces to the configurable-zone changes.
• Real-device validation exists, but more devices/configurations are welcome.
• Changing the touch-region depth can intentionally capture a larger edge area — that is why the setting is user-controlled.
• The Back indicator visuals themselves were not changed.