feat(highlight): replace the small-player highlight toggle with a strength slider (#4588)

## Description:

Replaces the "Highlight small players" on/off toggle with a **0-500%
Strength slider** controlling the glow's **width** (0 = off, 100% = the
previous default).

- `UserSettings`: `highlightSmallPlayers()`/toggle →
`highlightGlowStrength()`/set, float-backed, clamped to [0, 5].
- `SmallPlayerGlowPass` takes the strength via a setter (pushed by
`ClientGameRunner` on the settings-changed event), so it stays a pure
consumer and the slider still updates live while the settings modal has
the sim paused. Width = a non-linear blur-iteration count; intensity
compensated so wider stays about as bright.
- `MapRenderer` persists the strength across a WebGL context restore.
- `SettingsModal`: the toggle row becomes a range input.

Behavior note: with the toggle gone the glow **defaults ON at 100%**
(old toggle defaulted OFF); can default to 0 (opt-in) if preferred.

## Please complete the following:

- [x] I have added screenshots for all UI updates
- [x] I process any text displayed to the user through translateText()
and I've added it to the en.json file
- [x] I have added relevant tests to the test directory

resources/lang/en.json

@@ -1522,8 +1522,8 @@
     "ground_attack_desc": "Send a ground attack to the tile under your cursor.",
     "help_messages_desc": "Show contextual tips and warnings during gameplay, such as army limit warnings and general gameplay advice.",
     "help_messages_label": "Help Messages",
+    "highlight_glow_strength_label": "Highlight strength",
     "highlight_small_players_desc": "Add a glow to small players so they're easy to find.",
-    "highlight_small_players_label": "Highlight small players",
     "keybind_conflict_error": "The key {key} is already bound to another action.",
     "keybinds_hint": "Click a key to rebind it. You can assign a single key or Shift + key combination.",
     "left_click_desc": "When ON, left-click opens menu and sword button attacks. When OFF, left-click attacks directly.",