fix(doc): capture line-wrapped base64 in clipboard data URI regex

[fangshuyu-768]

Summary

Targeted fix for PR #508 addressing the review thread at #discussion_r3116012669: the strings.Fields normalisation added in adac5bf is a no-op, and line-wrapped base64 payloads are silently uploaded as corrupt 8-byte "PNGs".

Root cause

reBase64DataURI has character class [A-Za-z0-9+/\-_]+=*, which excludes whitespace. For a 76-char-folded HTML payload — standard MIME base64 folding, very common in real clipboards (Chrome, Safari, Feishu copy-image-as-HTML) — the regex stops at the first \n, so m[2] is already truncated by the time the strings.Fields strip runs. Fields then normalises a string that by construction has no whitespace, which does nothing.

Concretely, a payload like:

data:image/png;base64,iVBORw0KGgo
AAANSUhEUgAAAAE...

matches only iVBORw0KGgo (11 chars), decodes to exactly the 8-byte PNG signature 89 50 4E 47 0D 0A 1A 0A, passes hasKnownImageMagic, and ends up being uploaded as a truncated 8-byte "PNG" block. The user sees a broken image.

Fix

Extend the character class to include \s, so the existing strings.Fields strip on line 144 of clipboard.go actually has whitespace to normalise. Terminators (", <, ), ;) stay outside the class, so the match still ends at the URI boundary.

Test plan

• go test ./shortcuts/doc/... -count=1 passes (new TestReBase64DataURI_LineWrapped added)
• go vet ./shortcuts/doc/... clean
• gofmt -l clean
• The new test feeds a ≥180-byte payload folded with a mix of \n, \r\n, and \t through the regex, asserts full round-trip byte-equality after strings.Fields normalisation, and confirms the regex does not run past the "> attribute terminator (catches an over-permissive regression in the other direction).
• Existing tests — TestReBase64DataURI_Match, TestReBase64DataURI_URLSafeMatch, TestReBase64DataURI_NoMatch, TestExtractBase64ImageFromClipboard_WithFakeOsascript — still pass.

Notes

• Targets feat/media-insert-clipboard so this lands as part of PR feat(doc): add --from-clipboard flag to docs +media-insert #508 before it merges to main.
• The TIFF / PR-body inconsistency flagged in #discussion_r3116013106 is not addressed here — that one's a design call for @herbertliu (restore the TIFF middle layer vs. update the PR description).

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