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/app.css

@@ -14,6 +14,25 @@
   --text-tertiary: #6b7280;
   --border-color: #2a2a4a;
 
+  /* Trans pride colors */
+  --trans-blue: #5bcefa;
+  --trans-pink: #f5a9b8;
+  --trans-white: #ffffff;
+  --trans-gradient: linear-gradient(
+    135deg,
+    var(--trans-blue) 0%,
+    var(--trans-pink) 50%,
+    var(--trans-white) 100%
+  );
+  --trans-gradient-vibrant: linear-gradient(
+    135deg,
+    var(--trans-blue) 0%,
+    var(--trans-pink) 35%,
+    var(--trans-white) 50%,
+    var(--trans-pink) 65%,
+    var(--trans-blue) 100%
+  );
+
   /* Terminal specific colors */
   --terminal-user: #22d3ee;
   --terminal-tool: #c084fc;
@@ -44,6 +63,25 @@
   --text-tertiary: #9ca3af;
   --border-color: #d0d0e0;
 
+  /* Trans pride colors */
+  --trans-blue: #5bcefa;
+  --trans-pink: #f5a9b8;
+  --trans-white: #ffffff;
+  --trans-gradient: linear-gradient(
+    135deg,
+    var(--trans-blue) 0%,
+    var(--trans-pink) 50%,
+    var(--trans-white) 100%
+  );
+  --trans-gradient-vibrant: linear-gradient(
+    135deg,
+    var(--trans-blue) 0%,
+    var(--trans-pink) 35%,
+    var(--trans-white) 50%,
+    var(--trans-pink) 65%,
+    var(--trans-blue) 100%
+  );
+
   /* Terminal specific colors */
   --terminal-user: #0891b2;
   --terminal-tool: #7c3aed;
@@ -61,6 +99,55 @@
   --hljs-meta: #64748b;
 }
 
+[data-theme="high-contrast"] {
+  --bg-primary: #000000;
+  --bg-secondary: #0a0a0a;
+  --bg-terminal: #000000;
+  --bg-hover: #1a1a1a;
+  --bg-code: #0a0a0a;
+  --accent-primary: #ff4d6d;
+  --accent-secondary: #ff85a1;
+  --text-primary: #ffffff;
+  --text-secondary: #e0e0e0;
+  --text-tertiary: #b0b0b0;
+  --border-color: #ffffff;
+
+  /* Trans pride colors (high contrast) */
+  --trans-blue: #00d4ff;
+  --trans-pink: #ff99cc;
+  --trans-white: #ffffff;
+  --trans-gradient: linear-gradient(
+    135deg,
+    var(--trans-blue) 0%,
+    var(--trans-pink) 50%,
+    var(--trans-white) 100%
+  );
+  --trans-gradient-vibrant: linear-gradient(
+    135deg,
+    var(--trans-blue) 0%,
+    var(--trans-pink) 35%,
+    var(--trans-white) 50%,
+    var(--trans-pink) 65%,
+    var(--trans-blue) 100%
+  );
+
+  /* Terminal specific colors - bright and saturated */
+  --terminal-user: #00ffff;
+  --terminal-tool: #ff00ff;
+  --terminal-tool-name: #ffaaff;
+  --terminal-error: #ff5555;
+
+  /* Syntax highlighting colors (high contrast) */
+  --hljs-keyword: #ff66ff;
+  --hljs-string: #66ff66;
+  --hljs-number: #ffff00;
+  --hljs-comment: #aaaaaa;
+  --hljs-function: #ff99ff;
+  --hljs-type: #00ffff;
+  --hljs-variable: #ffaa00;
+  --hljs-meta: #cccccc;
+}
+
 html,
 body {
   margin: 0;
@@ -101,3 +188,52 @@ body {
   background: var(--accent-primary);
   color: var(--text-primary);
 }
+
+/* Trans gradient button - primary action buttons */
+.btn-trans-gradient {
+  background: var(--trans-gradient-vibrant) !important;
+  border: none !important;
+  color: #1a1a2e !important;
+  font-weight: 600;
+  text-shadow: 0 0 2px rgba(255, 255, 255, 0.5);
+  transition: all 0.2s ease;
+}
+
+.btn-trans-gradient:hover:not(:disabled) {
+  filter: brightness(1.1);
+  box-shadow:
+    0 0 20px rgba(91, 206, 250, 0.4),
+    0 0 30px rgba(245, 169, 184, 0.3);
+}
+
+.btn-trans-gradient:disabled {
+  opacity: 0.5;
+  cursor: not-allowed;
+  filter: grayscale(0.3);
+}
+
+/* Trans gradient focus border for inputs */
+.input-trans-focus {
+  position: relative;
+  transition: all 0.2s ease;
+}
+
+.input-trans-focus:focus {
+  border-color: var(--trans-pink) !important;
+  box-shadow:
+    0 0 0 1px var(--trans-blue),
+    0 0 12px rgba(91, 206, 250, 0.3),
+    0 0 20px rgba(245, 169, 184, 0.2) !important;
+  outline: none !important;
+}
+
+/* Trans gradient hover for icon buttons */
+.icon-trans-hover {
+  transition: all 0.2s ease;
+}
+
+.icon-trans-hover:hover {
+  color: var(--trans-pink) !important;
+  filter: drop-shadow(0 0 6px rgba(91, 206, 250, 0.5))
+    drop-shadow(0 0 10px rgba(245, 169, 184, 0.4));
+}