fix(watermark): align canvas rotation with PDF CCW convention

[gzz727007]

🦭 Pull Request Description

Summary of Changes

Watermark rotation was mirrored: setting -45° in the UI produced a watermark that looked like +45° (and vice versa).

Root cause: PDF user space is Y-up — positive angles rotate counterclockwise (ISO 32000; matches Adobe Acrobat). Canvas 2D is Y-down (screen raster), so a positive angle visually rotates clockwise. The raw user-facing angle was passed to ctx.rotate() without sign conversion, while the pdf-lib fallback branch (rotate: degrees(rotation)) already followed the PDF convention — the two render paths even disagreed with each other.

Fix (sign-only, 3 sites):

• src/tools/WatermarkTool.vue — live preview canvas: rotate((-wmAngle * π) / 180)
• src/tools/WatermarkTool.vue — export stamping canvas: same negation
• src/utils/pipeline/nodes/watermarkNode.js — pipeline Canvas branch: rotate((-rotation * π) / 180), now visually consistent with the degrees(rotation) fallback in the same file

After this change all paths share one convention: positive = counterclockwise, so -45° renders as \ and +45° as /, consistent with Acrobat and pdf-lib. No changes to query-param parsing (angle still clamped to -90..90) or stored settings.

Type of Change

• 🐛 Bug fix (non-breaking change fixing an issue)

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🛡️ Privacy & Quality Checklist

• 100% Client-Side Invariant: All code runs strictly in the browser RAM / WebAssembly. No user documents or telemetry data are ever sent over the network. (sign-only change; no network/IO code touched)
• Automated Tests: I have verified that all existing tests pass (npm test). (vitest: tests/watermark.test.js + tests/toolQueryParams.test.js, 24/24 passed)
• New Tests Added: Canvas raster path is not executable in Node (no canvas — the pipeline node falls back to the pdf-lib branch, which was already correct and is covered). Recommend a follow-up e2e visual assertion for angle directions.
• Responsive & Cross-Browser: Verified that UI works smoothly across desktop and mobile screens. (no layout/CSS changes; recommend a quick manual check of -45°/+45° preview before merge)
• Self-Review: I have performed a self-review of my own code and removed unnecessary console.log statements.

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Related Issues

Closes #

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Status	Name	Latest Commit	Preview URL	Updated (UTC)
✅ Deployment successful! View logs	pdfseal	01f305b	Commit Preview URL Branch Preview URL	Sep 29 2026, 01:40 PM