fix(learn): six Learn tab bug fixes

[ryanbas21]

Summary

• Reset learnCanvas on ClearFlow — stale selected node, pan position, zoom, and card positions were retained after clearing, causing phantom state when new events arrived
• Fix server card "— OK" label — when no network response was captured the server card showed "— OK" (misleading); now shows "No response"
• Fix expanded panel clipping — detail panels were hardcoded to 120px height, silently clipping SDK error details (6+ rows); increased to 200px with overflow-y: auto
• Fix zoom direction inversion — scroll down was zooming in (opposite of convention); inverted deltaY sign so scroll down = zoom out
• Remove dead LearnPan/LearnEndPan handlers — two Msg constructors that were never produced by any view code, removed from type and update function
• Neutral empty-state message — opening the Learn tab with no events showed DaVinci-specific copy; now shows a generic "No auth events recorded yet." message

Test plan

• All 142 Elm tests pass (pnpm dlx elm-test from packages/devtools-extension)
• Build passes (node build.mjs from packages/devtools-ui)
• Clear flow with an active Learn canvas node selected — canvas resets cleanly
• Select a DaVinci node with no network events captured — server card shows "No response"
• Click a card to expand it with 6+ detail rows — panel scrolls instead of clipping
• Scroll on the Learn canvas — scroll down zooms out, scroll up zooms in
• Open Learn tab with zero events — shows neutral empty-state copy

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