feat(misc): Add font color option for page numbers; improve alignment & robustness (#4334)

# Description of Changes

**What was changed**
- **API & backend**
- Added optional `fontColor` (hex, e.g. `#FF0000`) to
`AddPageNumbersRequest` with OpenAPI docs, default `#000000`.
- Decode hex color with safe fallback to black; apply via
`setNonStrokingColor`.
- Switched multiple `switch` statements to concise switch expressions
and used `Locale.ROOT` for case operations.
- Clamped `position` to `1..9` and reworked alignment using proper font
metrics (`ascent`/`descent`) for top/middle/bottom positioning.
  - Centralized filename base extraction; reduced repeated calls.  
  - Used try-with-resources for `PDPageContentStream`.
- **UI & i18n**
  - Added `addPageNumbers.fontColor` label (en_GB).  
- Introduced `<input type="color" id="fontColor" ...>` with live
background preview in the Add Page Numbers form.

**Why the change was made**
- Enable users to render page numbers in a chosen color (not just
black).
- Produce visually correct placement by accounting for font metrics
(baseline vs. optical middle).
- Improve resilience (locale-safe parsing, bounds checking) and code
clarity.

Closes #3839


[after.pdf](https://github.com/user-attachments/files/22064425/after.pdf)

[before.pdf](https://github.com/user-attachments/files/22064426/before.pdf)


---

## 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/devGuide/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)

### UI Changes (if applicable)

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

### Testing (if applicable)

- [ ] I have tested my changes locally. Refer to the [Testing
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/DeveloperGuide.md#6-testing)
for more details.

app/core/src/main/resources/messages_en_GB.properties

@@ -137,6 +137,7 @@ lang.yor=Yoruba
 
 addPageNumbers.fontSize=Font Size
 addPageNumbers.fontName=Font Name
+addPageNumbers.fontColor=Font Colour
 pdfPrompt=Select PDF(s)
 multiPdfPrompt=Select PDFs (2+)
 multiPdfDropPrompt=Select (or drag & drop) all PDFs you require

app/core/src/main/resources/templates/misc/add-page-numbers.html

@@ -99,6 +99,24 @@
                 <option value="Courier">Courier New</option>
               </select>
             </div>
+            <div class="mb-3">
+              <label for="fontColor" th:text="#{addPageNumbers.fontColor}"></label>
+              <div class="form-control form-control-color" style="background-color: #000000;">
+                <input type="color" id="fontColor" name="fontColor" value="#000000">
+              </div>
+                <script>
+                  let colorInput = document.getElementById("fontColor");
+                  if (colorInput) {
+                    let colorInputContainer = colorInput.parentElement;
+                    if (colorInputContainer) {
+                      colorInput.onchange = function() {
+                        colorInputContainer.style.backgroundColor = colorInput.value;
+                      }
+                      colorInputContainer.style.backgroundColor = colorInput.value;
+                    }
+                  }
+                </script>
+            </div>
             <div class="mb-3">
               <label for="startingNumber" th:text="#{addPageNumbers.selectText.4}"></label>
               <input type="number" class="form-control" id="startingNumber" name="startingNumber" min="1" required value="1">