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.

TODO.md

@@ -44,6 +44,11 @@ undefined identifiers, which is how
 
 # Completed Steps
 
+- 2026-08-11: A dust threshold of `0` now means "hide nothing" instead of
+  falling back to the 100,000 gwei default, and every address comparison in
+  `src/shared/transactions.js` goes through one case-normalising helper so a
+  checksummed genuine contract is no longer read as a spoof
+  ([#179](https://git.eeqj.de/sneak/AutistMask/issues/179)).
 - 2026-08-11: Policy compliance sweep — conditional verbose test rerun, local
   Tailwind binary instead of `npx`, `--frozen-lockfile` on `make install`, and
   the Makefile-only targets documented in the README