fix: honour a dust threshold of 0 and compare addresses case-insensitively (closes #179)

Two defects in the anti-poisoning filters, both silent over-filtering: the
wallet hid transactions the user had asked to see.

A dust threshold of 0 was read as `filters.dustThresholdGwei || 100000`, so
the one value a user would pick to mean "show everything" was swallowed and
replaced by the default. It is now `??`, making 0 a real threshold that hides
nothing and agrees exactly with clearing the hide-dust checkbox. The Settings
input rejects empty, negative, fractional and non-numeric entries outright
instead of coercing them, and resyncs the field to the stored value so it
never displays a threshold the wallet is not using.

isSpoofedSymbol compared the contract address with `===` against a lowercased
known address. EIP-55 mixed case is a checksum, not identity, so a genuine
token arriving checksummed was classified as a spoof and hidden. All address
comparisons in the module now go through one normalizeAddress helper, which
also normalises the fraud contracts recorded from a detected spoof.

Tests cover threshold 0 versus unset versus a set value, and the contract
comparison in lowercase, uppercase and EIP-55 form as well as against a
genuinely different address. The two `current behaviour:` tests pinning the
old behaviour are inverted into regression guards, and a fixture pairing a
real contract address with a null holder count pins the `tx.holders !== null`
guard that no fixture previously reached.

README.md

@@ -1108,7 +1108,8 @@ indexes it as a real token transfer.
   it. AutistMask hides transactions below a configurable dust threshold
   (default: 100,000 gwei / 0.0001 ETH). This is high enough to filter poisoning
   dust while low enough to preserve any transfer a user would plausibly care
-  about. The threshold is user-configurable in Settings.
+  about. The threshold is user-configurable in Settings; a threshold of `0`
+  hides nothing, exactly as clearing the checkbox does.
 
 - **User-configurable**: All of the above filters (known symbol verification,
   low-holder threshold, fraud contract blocklist, dust threshold) are settings