feat: Add share-link backend and wire up the sheet

Add a small Go + SQLite service (server/) that stores character-sheet
JSON blobs and returns short, content-addressed IDs, so sheets can be
shared via a compact ?s=<id> link instead of an oversized inline payload.

CharacterSheet.tsx now POSTs to the service on share and fetches by ID on
load, falling back to the self-contained ?c= inline link when the backend
is unreachable. Legacy ?c= links still decode.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

server/go.mod

@@ -0,0 +1,16 @@
+module github.com/drew/fabula-ultima-html/server
+
+go 1.26.4
+
+require (
+	github.com/dustin/go-humanize v1.0.1 // indirect
+	github.com/google/uuid v1.6.0 // indirect
+	github.com/mattn/go-isatty v0.0.20 // indirect
+	github.com/ncruces/go-strftime v1.0.0 // indirect
+	github.com/remyoudompheng/bigfft v0.0.0-20230129092748-24d4a6f8daec // indirect
+	golang.org/x/sys v0.42.0 // indirect
+	modernc.org/libc v1.72.3 // indirect
+	modernc.org/mathutil v1.7.1 // indirect
+	modernc.org/memory v1.11.0 // indirect
+	modernc.org/sqlite v1.52.0 // indirect
+)