Add media upload, clickable links, tables, comments, share

Media upload:
- Android file picker via ActivityResultContracts.GetContent
- Upload attachment API (base64 content to /api/v1/attachments)
- Uploaded attachments linked to memo via CreateMemoRequest.attachments
- Upload status shown in compose card ("uploading...", "N attachment(s) ready")

Markdown improvements:
- Clickable links: URLs and [text](url) open in browser via LinkAnnotation
- Table rendering: pipe-delimited markdown tables with header row
- Fixed underscore italic (_text_) and bold (__text__) variants

Comments:
- Comments section at bottom of MemoDetailScreen
- List existing comments via /api/v1/memos/{id}/comments
- Add new comments with text input + send button
- Comments rendered with full markdown

Share:
- "share" option in memo long-press context menu
- Android share intent with memo content as plain text

Also:
- Task toggle now works in memo feed (onTaskToggle callback wired)
- Tag clicks in explorer filter memos page
- Search from explorer filters memos page instead of navigating
- All three filter types (date/tag/search) shown as banner with "clear"

Co-Authored-By: Claude Opus 4.6 (1M context)

Signed-off-by: Avinal Kumar <avinal.xlvii@gmail.com>

androidApp/src/main/kotlin/com/avinal/memos/MainActivity.kt

@@ -19,6 +19,7 @@ class MainActivity : ComponentActivity() {
     override fun onCreate(savedInstanceState: Bundle?) {
         super.onCreate(savedInstanceState)
         deps.initialize()
+        com.avinal.memos.util.appContext = applicationContext
         enableEdgeToEdge()
         setContent {
             CompositionLocalProvider(LocalAppDependencies provides deps) {