fix(gui): fix identicon generation (#10228)

### Purpose

Identicon generation is supposed to consider the first 15 characters of
a device ID, e.g. if the ID is `ABCDEFG-HIJKLMN-...`, the identicon
should be based on `ABCDEFGHIJKLMN`.

However, the current implementation only strips the first dash,
resulting in `ABCDEFGHIJKLM-`, so the last character is essentially
fixed for all IDs. This corresponds to the lower-middle pixel in
identicons always being "off" (see screenshots below).

The fix is simple: Just add the `g` flag to the regex used to strip
dashes.

### Screenshots

Old vs. new, light theme:

<img width="130" height="55" alt="light-old"
src="https://github.com/user-attachments/assets/7aaf5a2d-bc8e-4fd9-af94-2e8d723e5369"
/>

<img width="130" height="55" alt="light-new"
src="https://github.com/user-attachments/assets/b91a789b-5dbc-4d46-99e0-84d89f634e1f"
/>

Old vs. new, dark theme:

<img width="130" height="55" alt="dark-old"
src="https://github.com/user-attachments/assets/c3de5d4b-83d6-41fa-98da-18dd720462d3"
/>

<img width="130" height="55" alt="dark-new"
src="https://github.com/user-attachments/assets/4d49f563-0ee6-40df-bae4-89149abf9e7e"
/>

Old vs. new, black theme:

<img width="130" height="55" alt="black-old"
src="https://github.com/user-attachments/assets/4c2e75a9-e658-4bdf-b9cc-e3dbc375cde3"
/>

<img width="130" height="55" alt="black-new"
src="https://github.com/user-attachments/assets/c321c3d7-7bbb-433f-b750-2a688956ea40"
/>

Only ~50% of identicons will be affected, since it's based on the parity
of the 15th character.

gui/default/syncthing/core/identiconDirective.js

@@ -34,7 +34,7 @@ angular.module('syncthing.core')
             middleCol = Math.ceil(size / 2) - 1;
 
             if (value) {
-                value = value.toString().replace(/[\W_]/i, '');
+                value = value.toString().replace(/[\W_]/g, '');
 
                 for (row = 0; row < size; ++row) {
                     for (col = middleCol; col > -1; --col) {