Display PDF loading progress in header toolbar

[allanlasser]

Fixes #547

I'm open to alternate placements, but this made sense to me: the viewer isn't fully interactive until the PDF loads.

[github-actions[bot]]

Coverage Report

Status	Category	Percentage	Covered / Total
🔵	Lines	23.61%	7454 / 31566
🔵	Statements	23.61%	7454 / 31566
🔵	Functions	41.03%	151 / 368
🔵	Branches	60.57%	418 / 690

File Coverage

File	Stmts	% Branch	% Funcs	% Lines	Uncovered Lines
Changed Files
src/lib/components/viewer/LoadingToolbar.svelte	0%	0%	0%	0%	1-34
src/lib/components/viewer/ReadingToolbar.svelte	0%	0%	0%	0%	1-159
src/lib/components/viewer/Viewer.svelte	0%	0%	0%	0%	1-119
src/lib/components/viewer/ViewerContext.svelte	0%	0%	0%	0%	1-193

Generated in workflow #621 for commit 48651b0 by the Vitest Coverage Report Action

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