fix: transaction subscribe source on websocket reconnect

[venimus]

Description

useWebSocket's reconnect handler (openWebSocket()) re-subscribes to a
transaction hash using source: "node", but the live watcher for the same
subscription (watch(subscribedTransactionId, ...)) uses source: "mdw".
Only source: "mdw" supports subscribing to an arbitrary tx hash payload —
source: "node" only handles the KeyBlocks/MicroBlocks channel names.

So after any websocket reconnect (network blip, backend restart) while a user
is on a transaction detail page, the re-subscription silently goes nowhere —
that transaction's live confirmation update never fires again until a full
page reload.

Demo

No visual change — this only affects a background websocket subscribe
message sent after reconnect.

Checklist:

• I have read and followed the Contributing Guide
• My change requires a change to the documentation.
• I have updated the documentation accordingly.

[github-actions]

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