Add cashtag support for stock ticker discussions (#9689)

* Add cashtag support for stock ticker discussions

Display cashtags ($TICKER format) as clickable links with dedicated menu actions and search feed, alongside hashtags. Includes composer highlighting and proper formatting.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

* Fix cashtag search query to use # prefix

Cashtags need to be searched as "#$BTC" rather than just "$BTC" to work
with the search API.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* Update @atproto/api to 0.18.14 with cashtag support

The updated package includes native CASHTAG_REGEX detection for stock
ticker symbols like $AAPL and $BTC.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* Fix todo, remove redundant validation

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Co-authored-by: Claude Haiku 4.5 <noreply@anthropic.com>
Co-authored-by: Eric Bailey <git@esb.lol>

package.json

@@ -226,7 +226,7 @@
     "zod": "^3.20.2"
   },
   "devDependencies": {
-    "@atproto/dev-env": "^0.3.196",
+    "@atproto/dev-env": "^0.3.204",
     "@babel/core": "^7.26.0",
     "@babel/preset-env": "^7.26.0",
     "@babel/runtime": "^7.26.0",