feat: add multiple productivity features and UI enhancements (#68)

## Summary

This PR adds a collection of productivity features and UI enhancements to improve the Hikari Desktop experience:

### New Features
- **Clipboard History** (#25) - Track and manage copied code snippets with language detection, search, filtering, and pinning
- **Quick Actions Panel** (#15) - Buttons for common quick actions like "Review PR", "Run tests", "Explain file", with customizable actions
- **Git Integration Panel** (#24) - View current branch, changed/staged files, quick git actions (commit, push, pull), and branch management
- **Session Import/Export** (#8) - Export conversations to JSON and import previously saved sessions
- **Snippet Library** (#22) - Save and reuse common prompts with categories and quick insert
- **Session History** (#14) - Auto-save conversations with browsable history and search
- **High Contrast Mode** (#20) - Accessibility theme with improved visibility
- **Minimize to System Tray** (#11) - System tray support with right-click menu

### UI Enhancements
- Trans-pride gradient theme applied across UI elements
- Copy button added to code blocks
- Linter formatting and eslint-disable comments for cleaner code

## Closes

Closes #8
Closes #11
Closes #14
Closes #15
Closes #20
Closes #22
Closes #24
Closes #25
Closes #34
Closes #35
Closes #36
Closes #37
Closes #69
Closes #70

## Test Plan

- [ ] Verify clipboard history captures code from code block copy buttons
- [ ] Verify clipboard history captures manually selected text from terminal
- [ ] Test snippet library CRUD operations and insertion
- [ ] Test quick actions panel with default and custom actions
- [ ] Test git panel shows correct status, branch, and performs git operations
- [ ] Test session history auto-save and restore
- [ ] Test session import/export roundtrip
- [ ] Verify high contrast mode provides adequate contrast
- [ ] Test minimize to tray functionality and tray menu
- [ ] Verify trans-pride gradient theme displays correctly in all themes

---
*✨ This PR was created with help from Hikari~ 🌸*

Co-authored-by: Naomi Carrigan <commits@nhcarrigan.com>
Reviewed-on: #68
Co-authored-by: Hikari <hikari@nhcarrigan.com>
Co-committed-by: Hikari <hikari@nhcarrigan.com>

src/lib/components/ConversationTabs.svelte

@@ -236,28 +236,32 @@
         </div>
       {/if}
 
-      {#if $conversations.size > 1}
-        <button
-          onclick={(e) => deleteTab(id, e)}
-          class="absolute right-1 top-1/2 -translate-y-1/2 w-4 h-4 flex items-center justify-center rounded hover:bg-[var(--bg-secondary)] opacity-0 group-hover:opacity-100 transition-opacity"
-          title="Close tab"
-        >
-          <svg
-            class="w-3 h-3"
-            fill="none"
-            stroke="currentColor"
-            viewBox="0 0 24 24"
-            xmlns="http://www.w3.org/2000/svg"
+      <div
+        class="absolute right-1 top-1/2 -translate-y-1/2 flex items-center gap-0.5 opacity-0 group-hover:opacity-100 transition-opacity"
+      >
+        {#if $conversations.size > 1}
+          <button
+            onclick={(e) => deleteTab(id, e)}
+            class="w-4 h-4 flex items-center justify-center rounded hover:bg-[var(--bg-secondary)]"
+            title="Close tab"
           >
-            <path
-              stroke-linecap="round"
-              stroke-linejoin="round"
-              stroke-width="2"
-              d="M6 18L18 6M6 6l12 12"
-            />
-          </svg>
-        </button>
-      {/if}
+            <svg
+              class="w-3 h-3"
+              fill="none"
+              stroke="currentColor"
+              viewBox="0 0 24 24"
+              xmlns="http://www.w3.org/2000/svg"
+            >
+              <path
+                stroke-linecap="round"
+                stroke-linejoin="round"
+                stroke-width="2"
+                d="M6 18L18 6M6 6l12 12"
+              />
+            </svg>
+          </button>
+        {/if}
+      </div>
     </div>
   {/each}