Tables: add reset column sizing option

Closes #9523.

Adds a Reset > Reset column sizing action that restores fixed widths and stretch weights authored through TableSetupColumn(). Automatically-sized columns return to content fitting. The existing all-columns content-fit action is now labeled consistently, and the tables context-menu demo shows mixed fixed/stretch sizing.

Verification: C++11 syntax-only compilation for imgui.cpp, imgui_tables.cpp, and imgui_demo.cpp; focused runtime assertions for fixed width, stretch weight, and auto-fit restoration. ASan/UBSan could not link because the local MinGW toolchain does not include those runtimes.