Add hotkey descriptions for Escape and Enter actions in HelpModal (#3382)

If this PR fixes an issue, link it below. If not, delete these two
lines.
Resolves #3064

## Description:

Adds Esc and Enter to the Help modal hotkeys section with consistent key
styling and descriptions:

- **Esc** – Shown with label; description: "Closes menu. Cancels unit
build preview."
- **Enter (↵)** – Shown with symbol and label; description: "Confirms
input. Confirms unit build preview."

Both keys use the same keycap style as existing hotkeys. Enter label
uses the standard symbol (↵) in `getKeyLabel()`. New strings are in
`en.json` under `help_modal.action_esc` and `help_modal.action_enter`.

<img width="723" height="427" alt="image"
src="https://github.com/user-attachments/assets/52fc168a-a850-4ac5-ba82-72f7672f476c"
/>


## 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
- [x] I confirm I have thoroughly tested these changes and take full
responsibility for any bugs introduced

## Please put your Discord username so you can be contacted if a bug or
regression is found:

.wozniakpl

resources/lang/en.json

@@ -89,6 +89,8 @@
     "hotkeys": "Hotkeys",
     "table_key": "Key",
     "table_action": "Action",
+    "action_esc": "Closes menu. Cancels unit build preview.",
+    "action_enter": "Builds unit under cursor",
     "action_alt_view": "Alternate view (terrain/countries)",
     "action_coordinate_grid": "Toggle coordinate grid overlay",
     "action_attack_altclick": "Attack (when left click is set to open menu)",