Issue #583: confirm before overwriting in the Save As dialog

[timmolter]

Fixes #583.

Right-click → Save As… silently overwrote an existing file, with no confirmation. This affected every chart type, not just PieChart as reported.

Cause

Swing's JFileChooser has no built-in overwrite confirmation — there is no setOverwriteConfirmation flag, and the JDK 21 javadoc has no method, field, or client property for it. showSaveAsDialog() went straight from APPROVE_OPTION to the encoder, which opens a truncating FileOutputStream.

Fix

• OverwriteConfirmingFileChooser — overrides approveSelection() to prompt when the target exists. This is the standard Swing idiom; the javadoc documents approveSelection as the hook the UI calls precisely so it can be intercepted. Declining returns without calling super, leaving the dialog open so a different name can be picked.
• resolveSaveTarget(File, FileFilter) — resolves the path that will actually be written.
• suffixOf(FileFilter) — reads the suffix off SuffixSaveFilter instead of comparing description strings ("*.jpg,*.JPG"), removing seven brittle literals.

The part that needed care

The check cannot run against the raw selection. Users typically type chart and pick the format from the drop-down, and the encoder appends the extension itself — so checking getSelectedFile().exists() would miss the most common case entirely.

Resolution therefore calls the same addFileExtension method the matching encoder calls. This is not incidental: BitmapEncoder and VectorGraphicsEncoder differ (< vs <= on the length guard, and only the bitmap one normalizes an existing extension to lowercase). Reimplementing the rule would have reintroduced the bug on edge cases.

Worth noting that JavaFX's FileChooser — which inherits confirmation "for free" from native dialogs — has open bugs for exactly this: JDK-8124315 (no confirmation on Windows when an extension filter is used and the extension is omitted) and JDK-8123775 (GTK ignores the extension when deciding). The platforms that got this for free still got the extension case wrong.

Testing

New XChartPanelSaveTargetTest — 5 cases covering extension-omitted, no-duplication, encoder agreement, and the null-filter fallback. No XChartPanel is constructed, per the headless-CI constraint. Full suite: 146 pass. The dialog behavior itself was verified manually.

Deliberately out of scope

• Did not migrate to ChartEncoder.saveChart. It is the modern non-deprecated API and would collapse the switch to one line, but its raster path is a plain ImageIO.write with no quality control, while the JPG branch uses saveJPGWithQuality(..., 1.0f). Switching would silently degrade JPEG output. Worth revisiting if ChartEncoder gains a quality parameter.
• showExportAsDialog() still overwrites CSV exports silently. Same class of bug, but it writes N files into a chosen directory, so a per-file prompt is the wrong shape and it needs its own design. Happy to file separately.

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