fix: handle partial GitHub/wallet connection states in ConnectPanel (#52)

[rafaio1]

Fixes #52.

• Map the full {GitHub × wallet} state matrix in ConnectPanel: no-auth/no-wallet, no-auth/wallet-local-only, auth/wallet-linked, auth/wallet-local-only, and auth/wallet-mismatch (from WalletContext never reconciles its cached address against Freighter's actual active account #71). Each state renders distinct, non-misleading UI with appropriate messaging.
• Gate wallet connection guidance when unauthenticated: show "Sign in with GitHub first" prompt instead of allowing a confusing local-only wallet state that has no backend association.
• Distinguish "wallet connected locally but not linked to profile" from "wallet fully linked" using user.stellarAddress as the durability signal — prevents false confidence that a local Freighter permission grant equals a server-side payout address.
• Surface wallet mismatch state (from WalletContext never reconciles its cached address against Freighter's actual active account #71 reconciliation) with clear reconnect instructions when Freighter's active account diverges from the linked address.
• Merge WalletContext never reconciles its cached address against Freighter's actual active account #71 (WalletContext reconciliation) into this branch so mismatch is available on the context; TypeScript compiles cleanly with npx tsc --noEmit.

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