feat: backport HeliBoard QoL features — HCESAR, edge-scroll, swipe-to-hide, HW-keyboard toolbar (#74)

[AsafMah]

Implements the cheap, isolated HeliBoard QoL backports from the #74 deep-dive. 4 commits, one per feature.

Features

1. HCESAR layout (6c026a1e) — adds assets/layouts/main/hcesar.json + registers it in donottranslate.xml (predefined_layouts + display name HCÉSAR; MAIN Latin layouts consult that registry, not just folder auto-discovery).
2. Touchpad edge-scroll acceleration (e38a323a) — PREF_TOUCHPAD_EDGE_SCROLL (default off). Holding a finger in the outer 10% of the touchpad auto-repeats cursor movement with acceleration (HeliBoard's EDGE_THRESHOLD_PERCENTAGE/TOUCHPAD_ACCELERATION_FACTOR ported into the fork's TouchpadView.java, using view dims since the fork has no keyboard-coordinate touchpad).
3. Toolbar swipe-down-to-hide (29519ede) — PREF_TOOLBAR_SWIPE_DOWN_TO_HIDE (default off). Swipe down on the strip/toolbar hides the keyboard (requestHideSelf); threshold dy>50dp & |dy|>|dx|, matching HeliBoard.
4. Only-toolbar-with-hardware-keyboard (683427a6) — PREF_SHOW_ONLY_TOOLBAR_WITH_HARDWARE_KEYBOARD (default off). With a HW keyboard attached, hides the on-screen key grid (mMainKeyboardFrame → GONE in KeyboardSwitcher) while keeping the toolbar/strip.

Each new pref follows the 5-file settings pattern + a SettingsContainerTest case.

Verification

• ✅ :app:testOfflineDebugUnitTest --tests "*SettingsContainerTest" → BUILD SUCCESSFUL (compiles clean; all 3 new prefs registered).
• ⏳ On-device (your call — these are view-layer/feel features the JVM can't exercise):
  • edge-scroll: hold a finger in the outer 10% of the touchpad → cursor auto-repeats with acceleration
  • swipe-down: swipe down on the suggestion/toolbar strip → keyboard hides
  • hw-keyboard: connect a BT/USB keyboard + enable the pref → on-screen key grid disappears, toolbar stays

All defaults are off, so nothing changes for existing users unless toggled.