Replace old 150-token list with 511-token tokenList.js

Delete src/shared/tokens.js and migrate all consumers to
src/shared/tokenList.js which has 511 tokens (vs ~150) sourced
from CoinGecko with on-chain verified decimals.

- prices.js: getTopTokenPrices now from tokenList
- transactions.js: KNOWN_SYMBOLS now from tokenList (3.4x more
  symbols for spoof detection)
- send.js: KNOWN_SYMBOLS for token dropdown filtering
- approval.js: uses pre-built TOKEN_BY_ADDRESS map instead of
  constructing its own from TOKENS array
- addToken.js: uses getTopTokens(25) for quick-pick buttons
  (only top 25 shown, not all 511)

src/shared/prices.js

@@ -1,6 +1,6 @@
 // Price fetching with 5-minute cache, USD formatting, value aggregation.
 
-const { getTopTokenPrices } = require("./tokens");
+const { getTopTokenPrices } = require("./tokenList");
 
 const PRICE_CACHE_TTL = 300000; // 5 minutes