Fix transparent dropdowns, icon-only header buttons, Kalam PDF

[ankitsejwal]

Three fixes from testing.

• Dropdowns were see-through. The shadcn Select/dropdown content uses bg-popover, but the --popover tokens were never defined in styles.css (only --card etc.), so the background resolved to nothing. Added --popover / --popover-foreground for both light and dark.
• Open / Save are icon-only now. The upload/download icons are self-explanatory; dropped the text labels and added tooltips.
• Kalam exported as gibberish. Tracked it down by rendering each font to a PDF: Kalam came out as scattered half-words ("Cortona" → "or o") while Patrick Hand / Shadows Into Light were clean. The culprit is pdf-lib's font subsetter dropping glyphs — embedding the full font (subset: false) fixes it completely. The size cost is trivial next to the 300-DPI photos, and only the one selected font is embedded per export.