[balabommablock-cpu]

Closes #711.

Adds two small arrows next to the time picker so you can step to the previous or next period without typing dates.

• Named periods snap to whole periods, so "This Week" goes back to the full week before (respecting your first day of week), and "This Month" to the full month before.
• Custom ranges step by their own length, or by calendar months if they cover whole months.
• The "next" arrow hides when the next period would start in the future.
• Added the two chevron icons to the icon bundle and rebuilt icons.dist.js and its .br copy.

Small note: on custom ranges at about 1280px wide, the longer date label wraps the Category filter onto a second row. This already happens on master without this change, so I left it out of scope. Happy to tidy it in a follow-up.

Tried to keep it slim and seamless, as asked. Less date-typing, more time-tracking. Happy to tweak anything.

[muety]

Thanks for your contrib. For the future, however, I'd appreciate if people could respect our contribution guidelines and first check back on an open issue before starting their implementation and PR right away.