Convert remaining ASCII box-drawing diagrams to mermaid across docs

[8bitpal]

Context

The project standard is to use fenced ```mermaid``` blocks for architecture / flow diagrams (per .internal/lessons-learned.md: "use ```mermaid fences").

PR #629 converted the ASCII box-drawing diagrams in the two new App Development curriculum modules (m4-dapp-sdk.mdx, m4-wallet-gateway.mdx) and in the closely related integrations/dapp-building-overview.mdx to mermaid. While doing the inventory it became clear that the same pattern persists in 14 other COPIED-marker pages that mirror upstream sources where the upstream still uses ASCII.

This issue tracks the broader sweep.

Convention to follow

PR #629 established the local convention for diverging from a COPIED-block upstream:

1. Replace the ASCII diagram block with a \``mermaid``` block (using flowchart LR / flowchart TB to match the existing project style — see m1-mental-models.mdx, overview/learn/ledger-model.mdx, sdks-tools/development-tools/pqs/operate.mdx for examples).

2. Add an MDX comment after the COPIED_END marker explaining the deviation:

   {/* COPIED_END */}
   
   {/* LOCAL_MODIFICATION: the "<diagram name>" diagram was converted from ASCII box-drawing to mermaid for consistency with other diagrams in the project. The diagram is the only divergence from the upstream source; the surrounding prose is verbatim. */}

3. Do not touch surrounding prose inside the COPIED block. The hash recorded in COPIED_START no longer matches upstream after the conversion; that's expected, and the LOCAL_MODIFICATION comment is the load-bearing signal for future refreshes that something has been edited locally.