feat: increase default weight of Bitter font for improved rendering (#1922)

## Summary

**What is the goal of this PR?**
* Provide a better default weight of Bitter font for improved rendering

**What changes are included?**
* Changes the default weight for Bitter font that is defined in the
custom font catalog to medium (500) from regular (400) for improved
rendering on e-ink, especially when font anti-aliasing is turned on

## Additional Context

* Bitter font at regular weight can become washed out when AA is on due
to some of the thinner stems and posts of the font. Bumping the weight
up to medium makes it sturdier and holds up better when AA is turned on.


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### AI Usage

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Did you use AI tools to help write this code? _**< NO >**_

lib/EpdFont/scripts/sd-fonts.yaml

@@ -108,9 +108,9 @@ families:
     intervals: latin-ext,cyrillic
     sizes: [12, 14, 16, 18]
     styles:
-      regular:    {url: "https://raw.githubusercontent.com/google/fonts/main/ofl/bitter/Bitter%5Bwght%5D.ttf", variable: {wght: 400}}
+      regular:    {url: "https://raw.githubusercontent.com/google/fonts/main/ofl/bitter/Bitter%5Bwght%5D.ttf", variable: {wght: 500}}
       bold:       {url: "https://raw.githubusercontent.com/google/fonts/main/ofl/bitter/Bitter%5Bwght%5D.ttf", variable: {wght: 700}}
-      italic:     {url: "https://raw.githubusercontent.com/google/fonts/main/ofl/bitter/Bitter-Italic%5Bwght%5D.ttf", variable: {wght: 400}}
+      italic:     {url: "https://raw.githubusercontent.com/google/fonts/main/ofl/bitter/Bitter-Italic%5Bwght%5D.ttf", variable: {wght: 500}}
       bolditalic: {url: "https://raw.githubusercontent.com/google/fonts/main/ofl/bitter/Bitter-Italic%5Bwght%5D.ttf", variable: {wght: 700}}
 
   # ── Sans-serif ─────────────────────────────────────────────────────────