Add configurable game/initial screen width and distance, with a bundled first-run config

- Video Options tab offers width and distance presets independently for
  the Initial Screen (splash/intro/menu) and Game Screen (gameplay)
  profiles, with their preset step/offset configurable via
  questshock.config.json.
- Ships res/questshock.config.default.json as the initial config on
  first run (or whenever questshock.config.json doesn't exist yet),
  refreshed from the APK on every launch without touching a user's own
  saved config.

CONFIG.md

@@ -9,9 +9,11 @@ startup, in addition to (and independent of) the engine's own `prefs.txt`.
 - **Format**: plain JSON, grouped by concern. Each top-level key is a
   *group*; each key inside a group is one *option*.
 - **Loading**: read automatically at app startup, before the splash screen,
-  if the file exists. If it doesn't exist, nothing changes - the engine
-  behaves exactly as it did before this file existed (driven by `prefs.txt`
-  as usual).
+  if the file exists. If it doesn't, `questshock.config.default.json` (same
+  folder) is used instead - a copy bundled with the app, refreshed on every
+  launch, never overwriting your own saved `questshock.config.json`. If
+  neither file exists, the engine behaves as it did before this file
+  existed (driven by `prefs.txt` as usual).
 - **Saving**: never automatic. In-headset, open the menu (controller menu
   button) and press **Save Configuration** to write the file from your
   current live settings, overwriting it if present. Press **Load
@@ -47,7 +49,15 @@ startup, in addition to (and independent of) the engine's own `prefs.txt`.
 {
   "video_options": {
     "renderer": "opengl",
-    "resolution": "1024x768"
+    "resolution": "1024x768",
+    "initial_screen_width_m": 1.6,
+    "initial_screen_distance_m": 2.0,
+    "game_screen_width_m": 2.8,
+    "game_screen_distance_m": 2.0,
+    "screen_width_offset_m": 1.0,
+    "screen_width_step_m": 0.6,
+    "screen_distance_offset_m": 1.0,
+    "screen_distance_step_m": 0.5
   },
   "general": {
     "language": "english"
@@ -78,6 +88,56 @@ The game's internal rendering resolution.
 - **Possible values**: `"320x200"`, `"320x400"`, `"640x400"`, `"640x480"`, `"1024x768"`
 - **Restart required?**: No (live-appliable)
 
+### `initial_screen_width_m`, `initial_screen_distance_m`, `game_screen_width_m`, `game_screen_distance_m`
+
+The virtual screen's physical width and distance from the player (both in
+meters) - the game shows two independently configurable screen
+"profiles", switched automatically based on what's currently on screen:
+
+- **Initial Screen** (`initial_screen_*`) - the splash screen, intro
+  cutscenes, and the game's own main menu.
+- **Game Screen** (`game_screen_*`) - actual gameplay (the 3D view,
+  automap, cyberspace, etc.).
+
+Switching happens automatically - no player action needed - the instant
+the game itself transitions between these (e.g. starting/loading a game,
+or returning to the menu).
+
+- **Type**: number (meters)
+- **Possible values**: the in-headset Video Options tab offers 5 presets
+  each - widths `1.0`, `1.6` (the original, pre-this-feature default),
+  `2.2`, `2.8`, `3.4`; distances `1.0`, `1.5`, `2.0` (the original,
+  pre-this-feature default), `2.5`, `3.0` - but a hand-edited value
+  doesn't need to match a preset exactly (it's clamped to a sane range -
+  width `[0.2, 10.0]`, distance `[0.5, 10.0]` - rather than snapped or
+  rejected; the Video Options tab just shows no button selected for an
+  in-between value).
+- **Restart required?**: No (live-appliable)
+- **Note**: the screen is always flat - there's no curve option.
+
+### `screen_width_offset_m`, `screen_width_step_m`, `screen_distance_offset_m`, `screen_distance_step_m`
+
+Reshapes the 5 preset buttons the Video Options tab offers for width and
+distance (above) - config-file-only, there's no in-headset control for
+these. Shared by both **Initial Screen** and **Game Screen** (both
+columns' buttons show the same 5 values per row) - this only changes
+what the buttons offer, not either profile's current value.
+
+Preset button `index` (0-4) shows `offset + step * index`. For example,
+a distance offset of `0` and step of `2` shows buttons `0`, `2`, `4`,
+`6`, `8`; the defaults below reproduce this feature's original launch
+presets exactly.
+
+- **Type**: number (meters)
+- **Default values**: width offset `1.0`, step `0.6` (buttons `1.0`,
+  `1.6`, `2.2`, `2.8`, `3.4`); distance offset `1.0`, step `0.5` (buttons
+  `1.0`, `1.5`, `2.0`, `2.5`, `3.0`).
+- **Restart required?**: No (live-appliable)
+- **Note**: not clamped - an unusual offset/step (e.g. negative step) is
+  accepted as-is. A resulting preset value outside the current value's
+  own clamp range (see above) still clamps normally when that preset
+  button is actually pressed.
+
 ## General (`general`)
 
 ### `language`