Bump to 0.3.1 with the upstream emote regression fix

Quick patch release. The previous commit on this branch is a
straight cherry-pick of Infi's upstream "Fix a regression from
API 15 updates" (ff899ff), which restores BetterTTV emote
deserialisation by switching the Emote and Top100 DTOs from
public fields to public properties so System.Text.Json picks up
the [JsonPropertyName] attributes correctly.

Without this, a fresh Hellion Chat install would happily fetch
emote JSON from BetterTTV but every entry would deserialise to
empty defaults, leaving the cache silently empty.

Manifest version bumps from 0.3.0 to 0.3.1 in csproj, the
HellionChat.yaml manifest plus its changelog, the custom-repo
repo.json (assembly version, testing assembly version and the
three download links), and the README version banner. The 0.3.0
changelog entry stays underneath in chronological order.

Build (Release) verified clean.

ChatTwo/HellionChat.yaml

@@ -40,6 +40,23 @@ tags:
   - Replacement
   - Privacy
 changelog: |-
+  **Hellion Chat 0.3.1 — Upstream emote regression fix**
+
+  Cherry-picks Infi's upstream commit ff899ff "Fix a regression
+  from API 15 updates" which changes the BetterTTV emote DTOs
+  (Emote and Top100) from public fields to public properties.
+  System.Text.Json under the API 15 toolchain only honours the
+  [JsonPropertyName] attribute on properties, so the previous
+  field-based version deserialised every fetched emote into empty
+  default values. Result: BetterTTV emotes were silently broken
+  on fresh installs. The fix is six lines and applies cleanly on
+  top of our defensive null-check from earlier; the EmoteCache
+  path-traversal hardening from 0.3.0 stays as it is.
+
+  Authorship of the fix is preserved with git cherry-pick -x, so
+  Infi shows up as the author on the commit. Thanks to him for
+  catching it in the upstream codebase.
+
   **Hellion Chat 0.3.0 — Audit hardening, brand sweep and rebrand of slash commands**
 
   This release closes the remaining audit follow-ups from the