web: reader link to the left, downloads into a "pobierz" disclosure

Finish the top-bar rework. The reader entry point now leads the controls
bar on the far left (it was in the right-hand cluster); settings stays in
the top-right cluster.

The flat "download: txt md pdf · kalendarz" strip becomes a single
"pobierz" summary that opens a small floating panel with two rows:
 - a month calendar with its own month + year <select> pickers (localised
   month names, both pre-selected from the shown date) and a PDF button
   that hits /calendar?month=YYYY-MM;
 - the shown day's readings as txt/md/pdf, labelled with the date.

The panel is absolutely positioned under the link so opening it doesn't
reflow the bar. indexData gains MonthOpts/YearOpts (a calOpt list); the
year picker is a small window around the shown year.

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