content: T20 — flashcard consistency pass for coding problems

[OpenSauce]

Bumps the submodule ref to pick up 59 new problem-anchored pattern/insight flashcards + 4 factual corrections from OpenSauce/dsa-flash-cards#35 (merged).

What landed in the submodule

• Data structures: 29 new cards covering all 28 DS coding problems
• Algorithms: 23 new cards covering 22 of 23 algorithms problems
• Dynamic programming: 7 new cards

Each card is atomic (one fact, one question), names the problem it anchors to explicitly, and binds to an existing lesson slug in the same category so the unlock pipeline works.

Review corrections applied

Four factual errors in card content were flagged by Copilot on the submodule PR and fixed before merge:

• Remove Nth Node From End: two-pointer description now specifies dummy head + n+1 gap so slow actually lands on the predecessor
• Sliding Window Maximum: monotonic deque invariant is "monotonic decreasing / non-increasing" to match the pop rule
• Find Minimum in Rotated Sorted Array: left-comparison guidance no longer overstates that it "fails" — clarified that right-comparison is just cleaner
• Word Break: O(k) cost attributed to substring creation/hashing, not hash-set lookup; noted the O(n²) path via trie or length iteration

Test plan

• Submodule PR chore(deps): bump actions/setup-node from 4 to 6 #35 CI green (lint + format)
• Parent validate check green (harness unaffected, content-only change)
• Parent backend / frontend checks green

[Copilot]

Pull request overview

Updates the dsa-flash-cards submodule reference to pull in the latest flashcard set (new problem-anchored pattern/insight cards plus a handful of factual corrections), keeping the parent repo’s content in sync with the upstream card source.

Changes:

• Bump dsa-flash-cards submodule pointer to include 59 new coding-problem-anchored flashcards.
• Include 4 upstream factual corrections to existing flashcard content.

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