Fix Windows scroll gaps and executable branding

[theincrediblewwz]

What changed

• keep mounted CodeMirror editors alive while the TanStack source list is being dragged, removing the stale static-height shell that could leave large blank tails inside blocks;
• add a real Electron regression that slowly drags the source scrollbar top-to-bottom-to-top four times and verifies both edges, Markdown byte identity, stable scroll height, and visible block-tail geometry;
• resource-edit the offline Windows packaging fallback and reject any final executable whose icon or version resources do not match MathNotes.

Root cause

The scrolling optimization could replace a mounted editor with a static shell whose cached minimum height no longer matched the current content. Separately, the offline packaging fallback copied and renamed electron.exe without running Windows resource editing, while the old brand check inspected only source strings rather than the packaged executable.

Validation

• Windows focused tests: 17/17
• Windows full suite: 493/493
• production Windows build and full Electron smoke: pass
• exact slow scrollbar gesture: 4/4 bottom touches and 4/4 top touches across 80 heterogeneous blocks; 80 Markdown hashes unchanged; 0 px settled height drift; 1 px maximum visible blank tail
• portable app launch: pass
• final executable: 7 icon resources match mathnotes.ico; MathNotes 0.2.0 version fields; byte hash differs from Electron
• public source release gate and private-machine reference scan: ready

No release assets, Actions artifacts, dependency caches, internal project documents, or private paths are included.