Scale graphics with text and add click-to-enlarge previews

[arvearve]

Embedded images and Mermaid diagrams previously kept fixed dimensions when document text size changed. This change scales graphics and image captions with the text, and lets readers click either graphic type to open a window-filling preview.

Closes #28.

Changes

• Scale inline graphics from their fitted baseline, preserving aspect ratio and capping them at the document column width. Graphics also shrink when text size decreases in a narrow window.
• Include text scale and content width in Mermaid snapshot cache keys so cached diagrams cannot restore an outdated size.
• Present image and Mermaid previews at document-window level, with Escape/Done dismissal and inactive document controls underneath. Preserve Mermaid zoom controls and load a higher-resolution local image thumbnail for the enlarged preview.
• Add QuickMD/test-image-scaling.md, an image-width regression test, and a standalone WebKit check using the actual Mermaid template.

Validation

• Local Debug build and all 187 XCTest cases passed:

  xcodebuild -project QuickMD/QuickMD.xcodeproj -scheme QuickMD \
    -configuration Debug -destination 'platform=macOS' \
    CODE_SIGNING_ALLOWED=NO test

• swift scripts/check-mermaid-scaling.swift passed proportional scaling, narrow-column shrinking, and preview-fit checks. Before the fix, Mermaid remained 356 points wide at 75%, 100%, and 150%; afterward it measured 267, 356, and 534 points.
• Checked PNG and Mermaid preview opening, dismissal, and Mermaid zoom controls in the local app.

Inline enlargement stops at the column width; clicking opens the preview using the available window space.

[b451c]

Merged locally with --no-ff (19a9ba1) and pushed to main - thank you, this is a well-built change: the scale-from-fitted-baseline rule, the cache keys that include scale and width, and the WebKit check script all held up under review and in an end-to-end run on the app (zoom 100 -> 125 -> 100, narrow window, previews via real clicks). Closes #28.

Follow-up on top of it (91aceb9), three things found in that run:

1. Mermaid snapshot taken before the frame caught up. A diagram that first rendered below the fold posted its height (70 pt) while the not-yet-placed cell still had the 200 pt default frame, so the snapshot was 646x200 (diagram plus blank space). Because the block now seeds diagramHeight from cached.size.height, every later re-creation of that block pinned the row at 200 pt (reproduced with log instrumentation: snapshot 646x200 frame=200 current=70). The coordinator now takes the snapshot only when webView.frame.height matches the reported height; the template's resize listener posts again once the frame catches up, and the snapshot fires then.
2. Focus ring bleeding through the preview: AppKit draws focus rings above every view, so the sidebar collapse button (first responder after launch with keyboard navigation on) kept its ring visible over the overlay. The document drops first responder before presenting.
3. The overlay used the system background; it now uses the document theme's background and text colour, so a dark custom theme under a light appearance no longer opens a white panel.

Also moved QuickMD/test-image-scaling.md into the local fixture set (test-files/ is where the manual fixtures live, next to their images) - the WebKit check script stays in scripts/. Will ship in the next release with credit in the CHANGELOG.

[b451c]

Shipped in v1.10.0: https://github.com/b451c/quickmd/releases/tag/v1.10.0 - credited in the CHANGELOG and release notes. Thank you!