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/stores/snippets.ts

@@ -0,0 +1,138 @@
+import { writable, derived } from "svelte/store";
+import { invoke } from "@tauri-apps/api/core";
+
+export interface Snippet {
+  id: string;
+  name: string;
+  content: string;
+  category: string;
+  is_default: boolean;
+  created_at: string;
+  updated_at: string;
+}
+
+function createSnippetsStore() {
+  const snippets = writable<Snippet[]>([]);
+  const categories = writable<string[]>([]);
+  const isLoading = writable(false);
+  const selectedCategory = writable<string | null>(null);
+
+  const filteredSnippets = derived(
+    [snippets, selectedCategory],
+    ([$snippets, $selectedCategory]) => {
+      if (!$selectedCategory) {
+        return $snippets;
+      }
+      return $snippets.filter((s) => s.category === $selectedCategory);
+    }
+  );
+
+  async function loadSnippets(): Promise<void> {
+    isLoading.set(true);
+    try {
+      const [snippetList, categoryList] = await Promise.all([
+        invoke<Snippet[]>("list_snippets"),
+        invoke<string[]>("get_snippet_categories"),
+      ]);
+      snippets.set(snippetList);
+      categories.set(categoryList);
+    } catch (error) {
+      console.error("Failed to load snippets:", error);
+    } finally {
+      isLoading.set(false);
+    }
+  }
+
+  async function saveSnippet(snippet: Snippet): Promise<boolean> {
+    try {
+      await invoke("save_snippet", { snippet });
+      await loadSnippets();
+      return true;
+    } catch (error) {
+      console.error("Failed to save snippet:", error);
+      return false;
+    }
+  }
+
+  async function createSnippet(name: string, content: string, category: string): Promise<boolean> {
+    const now = new Date().toISOString();
+    const snippet: Snippet = {
+      id: `custom-${Date.now()}-${Math.random().toString(36).substring(2, 9)}`,
+      name,
+      content,
+      category,
+      is_default: false,
+      created_at: now,
+      updated_at: now,
+    };
+    return saveSnippet(snippet);
+  }
+
+  async function updateSnippet(
+    id: string,
+    name: string,
+    content: string,
+    category: string
+  ): Promise<boolean> {
+    const currentSnippets = await invoke<Snippet[]>("list_snippets");
+    const existing = currentSnippets.find((s) => s.id === id);
+
+    if (!existing) {
+      console.error("Snippet not found for update");
+      return false;
+    }
+
+    const updated: Snippet = {
+      ...existing,
+      name,
+      content,
+      category,
+      updated_at: new Date().toISOString(),
+    };
+
+    return saveSnippet(updated);
+  }
+
+  async function deleteSnippet(snippetId: string): Promise<boolean> {
+    try {
+      await invoke("delete_snippet", { snippetId });
+      await loadSnippets();
+      return true;
+    } catch (error) {
+      console.error("Failed to delete snippet:", error);
+      return false;
+    }
+  }
+
+  async function resetDefaults(): Promise<boolean> {
+    try {
+      await invoke("reset_default_snippets");
+      await loadSnippets();
+      return true;
+    } catch (error) {
+      console.error("Failed to reset default snippets:", error);
+      return false;
+    }
+  }
+
+  function setSelectedCategory(category: string | null): void {
+    selectedCategory.set(category);
+  }
+
+  return {
+    snippets: { subscribe: snippets.subscribe },
+    categories: { subscribe: categories.subscribe },
+    filteredSnippets: { subscribe: filteredSnippets.subscribe },
+    isLoading: { subscribe: isLoading.subscribe },
+    selectedCategory: { subscribe: selectedCategory.subscribe },
+    loadSnippets,
+    saveSnippet,
+    createSnippet,
+    updateSnippet,
+    deleteSnippet,
+    resetDefaults,
+    setSelectedCategory,
+  };
+}
+
+export const snippetsStore = createSnippetsStore();