fix: update UI strings for improved CTAs, grammar, and consistency

[vytisbulkevicius]

Summary

• Grammar & typo fixes: hight→height, magnifiying→magnifying, maximun→maximum, "an user"→"a user", "it working"→"it work", "by change"→"by changing"
• Improved CTAs: "Add New"→"Add New Chart", "Insert"→"Insert Chart", "Click to select"→"Select Chart", "From Library"→"Insert from Library"
• Clearer labels: replaced developer jargon with user-friendly terms — "Interpolate Nulls"→"Fill in Missing Values", "Residue Slice"→"Other Slice", "Aggregation Target"→"Group Tooltips By", "Hex Value"→"Hex Color Code", "Dateless Region"→"Region with No Data", "Identical Direction"→"Default Direction", "From And To Range"→"Value Range (Min / Max)", "Family And Size"→"Font Family And Size"
• Reduced wordiness: simplified verbose descriptions (tooltip trigger, legend position, background color, stacked series)
• Better error messages: CSV upload errors and invalid URL messages now explain what went wrong and suggest next steps
• Actionable empty states: "No charts found" now includes guidance on what to do next
• Updated e2e tests to match new button labels

All changes are string-only — no logic, escaping, sanitization, or output changes. Translation wrappers and text domains preserved.

Test plan

• Verify chart library page shows "Add New Chart" button
• Verify empty library shows "No charts found. Click 'Add New Chart' to create one."
• Verify Gutenberg block shows updated "No charts found" message
• Verify media modal shows "Insert Chart" and "Insert from Library"
• Spot-check chart sidebar settings for updated labels (e.g., Pie chart "Other Slice Settings", Gauge "Value Range (Min / Max)", Line chart "Fill in Missing Values")
• Verify CSV upload error messages display correctly
• Run e2e test suite to confirm tests pass with updated strings

🤖 Generated with Claude Code

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