fix(viewer): dynamic page number input width based on total page count (#6607)

# Description of Changes


### Before:

<img width="3092" height="468" alt="image"
src="https://github.com/user-attachments/assets/8ed16ac3-9547-4f07-937e-4177c4ed16fd"
/>


### After: 

<img width="1868" height="490" alt="image"
src="https://github.com/user-attachments/assets/8ddf2227-bac4-49f6-973a-90c9f4667dfe"
/>


### Mobile (after):

<img width="842" height="444" alt="image"
src="https://github.com/user-attachments/assets/e601fa94-45a6-46ae-b432-550bb27a98a6"
/>



<!--
Please provide a summary of the changes, including:

- What was changed
- Why the change was made
- Any challenges encountered

Closes #(issue_number)
-->

---

## Checklist

### General

- [X] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [X] I have read the [Stirling-PDF Developer
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/DeveloperGuide.md)
(if applicable)
- [ ] I have read the [How to add new languages to
Stirling-PDF](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md)
(if applicable)
- [X] I have performed a self-review of my own code
- [X] My changes generate no new warnings

### Documentation

- [ ] I have updated relevant docs on [Stirling-PDF's doc
repo](https://github.com/Stirling-Tools/Stirling-Tools.github.io/blob/main/docs/)
(if functionality has heavily changed)
- [ ] I have read the section [Add New Translation
Tags](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md#add-new-translation-tags)
(for new translation tags only)

### Translations (if applicable)

- [ ] I ran
[`scripts/counter_translation.py`](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/docs/counter_translation.md)

### UI Changes (if applicable)

- [X] Screenshots or videos demonstrating the UI changes are attached
(e.g., as comments or direct attachments in the PR)

### Testing (if applicable)

- [X] I have run `task check` to verify linters, typechecks, and tests
pass
- [X] I have tested my changes locally. Refer to the [Testing
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/DeveloperGuide.md#7-testing)
for more details.

frontend/editor/src/core/components/viewer/PdfViewerToolbar.tsx

@@ -18,6 +18,12 @@ import ZoomInIcon from "@mui/icons-material/ZoomIn";
 import ZoomOutIcon from "@mui/icons-material/ZoomOut";
 import MoreVertIcon from "@mui/icons-material/MoreVert";
 
+// Sizing constants for the page number input
+const MIN_PAGE_DIGITS = 2;
+const MIN_INPUT_WIDTH_PX = 48;
+const BASE_INPUT_WIDTH_PX = 32;
+const PX_PER_DIGIT = 8;
+
 interface PdfViewerToolbarProps {
   // Page navigation props (placeholders for now)
   currentPage?: number;
@@ -131,6 +137,15 @@ export function PdfViewerToolbar({
     scrollActions.scrollToLastPage();
   };
 
+  const totalPagesDigits = Math.max(
+    MIN_PAGE_DIGITS,
+    (scrollState.totalPages || 1).toString().length,
+  );
+  const inputWidth = Math.max(
+    MIN_INPUT_WIDTH_PX,
+    BASE_INPUT_WIDTH_PX + totalPagesDigits * PX_PER_DIGIT,
+  );
+
   return (
     <Paper
       radius="xl xl 0 0"
@@ -195,10 +210,13 @@ export function PdfViewerToolbar({
         hideControls
         styles={{
           input: {
-            width: 48,
+            width: inputWidth,
             textAlign: "center",
             fontWeight: 500,
             fontSize: 16,
+            paddingLeft: 4,
+            paddingRight: 4,
+            boxSizing: "border-box",
           },
         }}
       />