fix(desktop): inset macOS and Linux app icons to platform icon grids (#7646)

# Description of Changes

The macOS Dock icon renders noticeably larger than every other app. The
cause is that
`icon.icns` was **100% full-bleed** - the red rounded square filled all
1024x1024 with zero
margin. macOS does not mask or inset legacy `.icns` icons, so the
artwork has to carry Apple's
grid itself: an **824x824 body centred on a 1024x1024 canvas**.
Full-bleed therefore rendered
**24% wider and 54% larger in area** than its neighbours.

Linux had the same defect for the same reason - the hicolor PNGs were
94.9-100% full-bleed,
and GNOME's HIG says an app icon is drawn within the canvas but must not
fill it (~10% margin,
so a body around 80%). Those small existing margins were resampling
artifacts, not padding.

Windows is deliberately **left full-bleed**. Microsoft imposes no inset:
target-size assets are
drawn without tile padding and the taskbar simply scales the bitmap into
the slot. `app.ico` is
a pure rename here, byte-identical to before.

## What changed

Icons are now split per platform, since the three platforms disagree
about how much of the
canvas the artwork may fill:

| Path | Owner | Treatment |
| --- | --- | --- |
| `icons/macos/app.icns` | `dmg`, `app` | 824/1024 Apple grid |
| `icons/macos/app-512.png` | build-time only | see note below |
| `icons/linux/app-{16..512}.png` | `deb`, `rpm`, `appimage` | ~10%
margin, KDE's small-size exception at 16/32 |
| `icons/windows/app.ico` | `msi`, NSIS | unchanged, full-bleed |

Linux is selected by a new `tauri.linux.conf.json`. Tauri merges
platform configs with
JSON Merge Patch (RFC 7396), so `bundle.icon` is **replaced wholesale**
rather than appended.

## Notes for reviewers

- **`icons/macos/app-512.png` is build ballast, not a real asset.**
`tauri-codegen` requires a
PNG in the icon list for every non-Windows target, with a hardcoded
fallback to
`icons/icon.png` - a file this PR deletes. Without it the build fails.
It is embedded as
`default_window_icon`, which tao's macOS backend discards
(`set_window_icon` there is a no-op:
  "macOS doesn't have window icons"). Nothing renders it.
- **Linux icon order matters.** The bundler derives the hicolor
directory from each PNG's real
pixel dimensions, so `app-128.png` lands in `128x128/`. `app-512.png` is
listed first because
the first PNG in the list also becomes the window icon, which GTK does
honour.
- **`.imgbotconfig` had to be repointed.** Its previous entry named
`icons/icon.png`, a path this
PR deletes. That exclusion is load-bearing: ImgBot once optimised the
icon to an indexed
palette and `tauri::generate_context!()` rejects non-RGBA icons,
breaking the desktop build
(#6990). All 15 generated PNGs, including the eight inside the `.icns`,
are verified colour
  type 6.
- **Not fixed here:** our corner radius is 14.3% of the body where macOS
and GNOME neighbours sit
near 22%, so the icon still reads squarer than its neighbours. That is a
brand-silhouette
  decision rather than the sizing bug, so it was left alone.
- The 15 pre-existing unused assets (`Square*Logo.png`, `mstile-*`,
`android-chrome-*`,
`android/`, `ios/`) are untouched. No configured bundle target consumes
them.

## Verification

`task check` was **not** run - this PR touches no Java, TypeScript or
engine Python, so it
cannot exercise the change. What was verified directly instead:

- Simulated the RFC 7396 merge and Tauri's `find_icon` resolution per
platform: Windows resolves
to `app.ico`, macOS to `app.icns` plus the stub PNG, Linux to its own
six PNGs. Every path exists.
- Both configs validate against the bundled
`@tauri-apps/cli/config.schema.json`, base and merged.
- Every PNG's real dimensions match its filename, and every body
measures exactly its nominal
  inset (410/512, 154/192, 102/128, 52/64, 28/32, 14/16).
- An overlay diff of the new macOS body against the old artwork shows
only 1px antialiasing
  hairlines - the mark itself is unchanged, only inset.
- Pre-commit hooks pass.

---

## 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)

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

### Testing (if applicable)

- [ ] 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.

.imgbotconfig

@@ -1,5 +1,7 @@
 {
   "ignoredFiles": [
-    "frontend/editor/src-tauri/icons/icon.png"
+    "frontend/editor/src-tauri/icons/macos/*",
+    "frontend/editor/src-tauri/icons/linux/*",
+    "frontend/editor/src-tauri/icons/windows/*"
   ]
 }

frontend/editor/src-tauri/tauri.conf.json

@@ -29,14 +29,9 @@
     "publisher": "Stirling PDF Inc.",
     "targets": ["deb", "rpm", "appimage", "dmg", "app", "msi"],
     "icon": [
-      "icons/icon.png",
-      "icons/icon.icns",
-      "icons/icon.ico",
-      "icons/16x16.png",
-      "icons/32x32.png",
-      "icons/64x64.png",
-      "icons/128x128.png",
-      "icons/192x192.png"
+      "icons/windows/app.ico",
+      "icons/macos/app.icns",
+      "icons/macos/app-512.png"
     ],
     "resources": ["libs/*.jar", "runtime/jre/**/*"],
     "fileAssociations": [

frontend/editor/src-tauri/tauri.linux.conf.json

@@ -0,0 +1,12 @@
+{
+  "bundle": {
+    "icon": [
+      "icons/linux/app-512.png",
+      "icons/linux/app-192.png",
+      "icons/linux/app-128.png",
+      "icons/linux/app-64.png",
+      "icons/linux/app-32.png",
+      "icons/linux/app-16.png"
+    ]
+  }
+}