fix: correct dark theme colors in encrypted vault delete confirmation dialog

[wired-pasteque]

Fix: #2257

[claude]

Claude finished @wired-pasteque's task in 1m 32s —— View job

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Review: dark theme fix for delete confirmation dialog

• Gather context (diff, file)
• Analyze color/theme changes
• Check for regressions in both bottom sheets
• Post review findings

Verdict: looks correct ✅ — the fix resolves #2257 and I found no correctness, security, or regression issues.

Why the fix is right

The root cause is a token mismatch. Bottom-sheet text (BBText / context.font.*) inherits the ambient theme text color, which is light in dark mode. The old background token resolved to white in both themes, so light text on a white sheet was invisible:

token	light	dark
onPrimary (old bg)	#FFFFFF	#FFFFFF ❌ light text on white
onSecondary (new bg)	#FFFFFF	#15171C ✅ light text on near-black

Switching the container background to context.appColors.onSecondary (drive_vaults_list_page.dart:99 and :148) gives a dark sheet in dark mode while preserving the white sheet in light mode, so the title/confirmation text and buttons are readable in both. The button contrast also holds up:

• Cancel/Export: bgColor: secondary / textColor: onSecondary — inverts cleanly against the new background in both themes.
• Delete: bgColor: primary (red) / textColor: onPrimary (white) — unaffected, readable in both.

Applying the same change to both sheets (_showActionsBottomSheet and _showDeleteConfirmationBottomSheet) is the right call — the actions sheet had the same latent dark-theme issue.

The remaining diff (Radius.circular(32) → 12, and the large reformat block) is cosmetic / dart format output with no functional impact.

Minor (non-blocking) nit

Using onSecondary as a container background is a little unconventional — onX tokens are semantically intended for foreground content drawn on X. It works here only because its values happen to be surface-appropriate (white / near-black). A token like surface / surfaceContainer would read more idiomatically for a sheet background, but the current choice yields the intended high contrast against the secondary-colored buttons, so it's a defensible deliberate choice rather than a bug. Worth a quick design sanity-check that #15171C (vs surface #1C1C1E) is the intended sheet shade.

Note: I reviewed by reading the diff and theme palette (lib/core/themes/colors.dart); I did not run flutter analyze in this environment. Changes are value swaps + formatting only, so analyzer risk is low.

[ethicnology]

@thibistaken and @i5hi should we make this part of #2258 ?